I don’t think the ending of Killer Killer was meaning to say they had actually died together, like he was saying they would commit suicide for the rest of their lives was his way of saying he would live out the rest of his natural life by her side. Honestly the romance felt unnecessary.
I actually really liked the romance! It's one of the only DR stories that had a canon romantic relationship between major characters. It was teased since the beginning, so it didn't feel like it got there out of nowhere. I actually thought it'd go the "teasing" route where two characters grow close but never get anything canonized, that was a pleasant surprise.
Anyways, I prefer to take it at face value. Takumi was mentioning his love for murder since the beginning, he chose to kill Misaki because of that. The passion thing would also apply to himself, and, well, since he kills murderers, and he is a murderer, the suicide part is justified too. Plus, it really works with that trope of lovers or otherwise close people deciding that they wouldn't want to live without others.
I feel like it also adds to Misaki's characterization. She feels more... Not timid, but kinda soft in a way- but then rather than cowering around and trying to hide, she asks him to kill her- and even though the conflict was resolved, when he comes back she still is willing to leave the world with him. And since it's a lover *suicide* - that suggests that she took an active part in it.
An ending where they fake their deaths would feel pretty much against their personal ideals, and a bit too... escapist? In a way. I don't think it's nearly as symbolic as the ending where they both die.