PREMISE: This is D&amp;D 5e.

A friend of mine runs a D&amp;D 5e game. Last session, we went up against a sleeping young red dragon, and while pondering what to do with it because while it wasn't our mission, the guy was standing in our way and we had the option to kill it or leave it be and possibly have to face it later, and we all collectively took the decision to jump the gun and face possible consequences later. All except for one of our party members, Aglet. (Bladesinger wizard)

Enter our surprise round, where we all beat up the dragon, the health visibly whittling down its health (the DM had messed up and let us see on roll20 the healthbar of the thing)... except Aglet, of course. Aglet uses his surprise round to Message my character asking, **while in the middle of a fight**, what the hell do we think we're doing.

For context, our two characters have a shared background, coming from a world different from the one of the setting where no dragons existed, and the only examples of dragons we had seen were either feral or not actual dragons but gods in disguise. So no "ambigously moral" to jump a creature that the party members had told us to be inherently evil. 

Still, we're the luckiest mofos on the face of the planet, and the dragon rolls a 1 on the initiative, allowing us a second round in a row.

We keep punching the dragon, he finally gets the hint that he should attack the dragon and does so. Over the course of the second round, the Dragon falls under exactly the amount of health that the dragon would've died with another Ray of Frost. Or with a stab from a knife. Or if you looked at it a bit too mean, really.

Still, alive is alive and the creature decides that adventurers Flambè is quite nice and uses its breath weapon. Do note that we're a semi-weary party of level 5 adventurers, so it's perfectly within the realm of possibility to get oneshot despite getting the save right.

Thank fuck our DM decided that our Rogue's critical dexterity save, plus his uncanny dodge and the pally's successful save were enough to make the both of them move out of the way, although knocking the pally prone because the rogue shoved her out of the way of 16d6 worth of fire damage.

With a collective phew of the group, it's our Wizard's turn to act. Since he was so adamant to not want to kill the thing, perhaps he can try and do nonlethal damage?

Nope. Ray of frost. Ironically ends up missing and the pally gets her revenge kill on the creature.

But after that. Hoo boy. Does he say "sorry for not attacking with you guys", both in game and out of game? Does he instead role it and ask pally and rogue if they're alright because they just avoided near certain doom, especially because pally is his in game SO?

Nope.

"You guys should have listened to me when I said I didn't want to attack it." 

Cue Rogue flipping his shit, telling him off. He can't do that because Aglet goes psycho and launches a ray of frost "of warning" (aka he missed). And then. And then.

"I roll Investigation to loot the dragon." we're kinda mad that he wants to do that when he did jackshit while we all wasted resources to take it down, but he rolls and dice gods be on our side, he finds nothing of use among its bones. And then he just leaves to wander off in the rest of the cave. 

The only justification he gives after being told off by yours truly out of game for

1 Not going through with a decision made by the whole party except him, which is honestly the least bad

2 Putting a party member in danger deliberately like that 

3 Not saying sorry and even claiming himself as the one with the moral high ground

4 Not being concerned at all with the fact that he nearly offed his spouse

5 Demanding loot after doing nothing to kill the dragon

and 6, attacking a fellow party member

and he just says, mr. Neutral Good, "It's what my character would've done."

to sum it up, Am I The Asshole for getting mad?