You should be strongly considering leaving this game—but over the other PC killing your character while you were away, far more than any concerns about an evil alignment being allowed or not. But as we often say, “no gaming is better than bad gaming.”
A group in which a fellow PC kills your character while you’re not even there sounds to me like it is always going to be bad gaming. And, for that matter, your head-butting over the alignment here suggests that this, too, is going to be a source of friction, even though it’s by a large margin the lesser issue.
To get it out of the way, ultimately, no one is forced to play the game, which is to say, no one is forced to play the game a particular way. The DM is not required to run a game they don’t wish to run, and a player is not required to play a game that they don’t wish to play. There is no other rule that takes precedence over this reality. So if your DM refuses to play a game with an evil PC, you can’t make them do so. If you refuse to play a game where you cannot be an evil NPC, they can’t make you, either.
What the books do or do not say is completely irrelevant at this point.
Because DMs tend to be scarcer than players, because DMing tends to be more work than playing, DMs usually have greater leverage in this regard, and for the record, the books largely are on their side. The Adventurers League does put some rules on the DM, but in non-AL play the books give the DM pretty much free reign. And, for that matter, the Adventurers League bans most evil alignments (you can only be Lawful Evil and then only when you belong to one of a couple of specific factions, which requires a particular background).
As the Adventurers League example shows, banning evil PCs is pretty common. Rather wrong-headed, in my opinion, because all alignments have serious risks of disrupting the game, and evil isn’t necessarily special in this regard, but nonetheless, it’s a pretty common thing to do. As a player, again, the only viable choice you get to make about this is to play or not play. That’s it. You cannot force the DM to allow something they do not want to allow.
Which brings us back around to the question of whether or not you should play: I’m pretty confident that the answer is no.
Banning evil-aligned PCs would be an irritation for me, as a player, and possibly a red flag that the DM either has fundamentally flawed beliefs about alignment in the first place or else simply doesn’t have a strong understanding of how it works out in practice, but nothing I’d quit a game over. But a PC killing another PC while the victim’s player is absent? No, sorry, thank you, but I have got better things to do with my time. That this happens suggests that the DM is, frankly, awful, and that the rest of the group is right there with them. I would want absolutely nothing to do with this group.
Real life happens. People cannot make every session. People cannot, even, necessarily give advanced warning that they will not make a session. As much as possible, you should make every effort to attend, and if you cannot, to give as much warning as possible—that’s only courteous, and regular failure to do so is a damn good reason to ask someone to leave the group. But it’s an out-of-game problem, that has to be addressed with an out-of-game solution. If they had a problem with you missing a session, that should have been a discussion they had with you—not something they “punished” you for in-character. “Punishment” has no place in a game. Anyone who thinks it does, is someone you don’t want to game with.
So I suggest you just give up on trying to make this DM happy, and try and find another game if you can. No need to fight about it, or attack them, or blame them—just a simple “you know, I think our ideas of what a game should be like are too different, and I don’t think this game is a good match for me or that I am a good match for this game. Best of luck, but I am bowing out.”