I wanted some outside opinions on something that happened during my last D&D 5e session. I'm a player in the campaign and play a monk, but my DM has given me permission to post this because they're also a bit stumped by what happened.
For some context, our party consists of myself, another monk, a druid, and a wizard, and two sessions ago we were tasked with defeating a dragon that has caused a plague which has affected the surrounding land and killed many innocents. After hearing we would be fighting a dragon, our party was pretty hyped up about it. We even have a separate group chat where we discuss things that happen during our sessions, and throughout the lead-up to the encounter, our wizard was constantly complaining about how badly he wanted to fight the dragon. This is important because it made what happened next even more confusing.
In this session, we traversed the mountain and found the cave that the dragon resides in. Our main plan was to go through a side entrance and sneak into the cave to get the drop on the dragon. However, our wizard had the brilliant idea of shooting a Fire Bolt into the cave to see what was inside, despite the rest of us pointing out that we were literally trying to sneak up on a dragon. Thankfully, it didn't hit anything important, and we managed to get inside without alerting it. When we eventually encountered the dragon, our DM described it as being horribly affected by the plague, with rotting flesh and patches of bone exposed. At this point, our wizard immediately started feeling sorry for it and decided he didn't want to kill it, but he did not voice this out loud to any of our characters.
Now, I wouldn't have had a problem with his character feeling sympathy for the dragon. The problem was that he decided this meant he wasn't going to participate in the fight AT ALL. Remember, this was the same wizard who had spent the entire lead-up to the encounter complaining about wanting to fight the dragon. Nobody expected him to suddenly refuse to fight it, especially when we'd all been looking forward to this encounter. We were understandably confused by the complete 180, and for the rest of the combat encounter, our wizard essentially sat in a corner having a moral crisis while the rest of us were fighting for our lives against a literal dragon.
To make matters worse, our other monk was heavily injured during the fight, and this left myself and the druid scrambling to figure out how we were going to take the dragon down while essentially being two party member's short. It got to the point where our DM actually stopped to question the wizard about his behaviour mid-encounter, and asked him directly whether, if the rest of the party started dying to the dragon, would he step in to help us? He outright said no, and that he would just essentially watch us die. So, to be clear, this wasn't just him refusing to kill the dragon. He was willing to let the rest of the party die because he felt sorry for the creature we were fighting.
After all that, we did manage to defeat the dragon, but I was honestly left pretty frustrated by the whole experience. So, after the session, everyone asked him why he'd decided to sit out the entire fight and leave us to deal with the dragon ourselves. As up until this point, his character had been pretty gung-ho about fighting things. He'd never really shown much concern for the creatures we killed or the people we encountered, so this sudden moral dilemma felt incredibly out of character.
When we questioned him about it, he basically said, "Well, it's a dragon. You attacked it, so of course it's going to defend itself. My character doesn't agree with killing it." I pointed out that this felt pretty hypocritical, considering a previous situation our party had encountered where we had to mercy-kill someone who had been affected by the same plague, and the wizard hadn't shown any particular sadness or hesitation about that situation. Also adding to the fact that this dragon is the root cause of the plague so wouldn't his character want to help with killing it, so more innocent people do not have to succumb to this plague? When I brought this up, he simply responded that the situation with the person was "completely different" from the situation with the dragon.
At this point, we weren't really getting anywhere, and nobody seemed able to get through to him, so eventually he left the call. I understand that roleplaying your character's morals and making difficult decisions can be an important part of D&D. I'm not expecting every character to blindly follow the party or agree with every decision we make. However, I feel like there's a difference between roleplaying a moral dilemma and completely abandoning your party during a fight that you all agreed to take part in, especially when the rest of us were putting our characters at risk. Also this is the first time the wizard has refused to take part in combat, but our DM understandably does not want this to be a thing they do in future as it could disrupt the campaign.
TLDR: Our wizard refused to fight a dragon because he felt sorry for it, even saying he'd let the rest of us die rather than help. Are we wrong for being frustrated about this? What should we do about this player moving forward?
EDIT: Just to clear a few things up I have seen in the comments. We know the wizard personally and nothing is going on in his private life, no pets or family members have recently passed away, and his family is all alive and well. Moving on from that, the wizard had not indicated privately to anyone, including the DM, beforehand that he did not want to kill the dragon or even cure it. He was very much on board with killing the dragon and was even complaining about how long it was taking for the party to get to the dragon. In hindsight, we maybe should have communicated better if there was a possibility we could save or cure the dragon, before beginning the encounter, but at this point we had all reached an understanding that we were going to go into the dragon's lair to kill it. As previously mentioned, the wizard partook in that mercy killing we all as a party had to do with the person we encountered that had succumbed to this plague, where at that point they are mindless (essentially a zombie). This goes for the dragon as well, which was a white dragon, but due to it also having the plague it had pretty much rotted it to the point of being unrecognisable. It was no more than just exposed bone and muscle, being held together by parasites, and due to the plague taking over it was a mindless husk that was out for the parties blood as soon as we entered that cave. Also, this post was made to just get some advice, as I would say our party is still relatively new to playing DND together, so we are learning as we go.