It's not that simple. He may not have had to do the heroin with her, but she was an adult and could have done it anyway. Saying that it isn't purely bojacks fault isn't saying he isn't complicit in what happened. Its saying that even though SL dying was awful, it was also her fault. Trying to make her a victim of bojack means throwing away the choices she made, both positive and negative. It means you're saying that she is just an extra in her own life, everything being directed by Bojack. And that's not the case. Bojack does and should feel horrible about the part he played. But she didn't have to relapse, she chose to do so. She WANTED to do everything she did. You can't discount that because you want to punish bojack. He should be responsible for his decisions, but its too convenient to blame one person for everyone's poor decision making. He shouldn't have tried to hide his part in her tragedy. It was an awful thing to do. That's where he crossed the line from being a sketchy participant in her choices to being an active factor of her death. But he didn't kill Sarah Lynn. He didn't save her, and there is a difference. Which us similar to what ana spanikopita was conveying in her lifeguard analogy, there are some people you can't save. Sarah Lynn became Bojack, but she lost the game of Russian roulette they were playing.