AITAH for unknowingly introducing my late sister to her murderer?

This is a throwaway for obvious reasons with different names and times because you can find photos of this on the internet and I want some privacy. This is gonna be horrific and gore so if you don’t like that just leave now.

About 11 years ago when I was 17 I brought home this friend I had Kevin, we’ve been friends for years but I just never brought him home like that, for no particular reason tho at first he was cool, he met my parents and my sisters, I had two sisters Emily who was 2 years older than me and Kate who’s 5 years younger, I could immediately tell he had a crush on my older sister, everyone did she was really pretty back then, fast forward a couple of months and I found out they were actually dating each other, I was pissed. I told Kevin it’s not cool and he told me it’s not any of my business and that I don’t own her, I went to Emily and she told me the same, even my parents told me it wasn’t my business, the age of consent here is 16 and he was 18 back then so it was also legal, I thought it’d be just a phone that’ll go away but it didn’t, they just stayed together through college and he eventually married my sister and they had two kids together.

At that point I was fine with it, he clearly loved my sister and wasn’t just fooling around with her and back then he was genuinely a good guy, but fast forward a couple of years ago and Kevin had a bad accident. After that his personality changed, he became abusive to my sister and to the kids, he just became a different person, he hit my sister and even sexually assaulted her multiple times, me and dad got involved multiple times and we beat his ass because she’d beg us not to call the police on him. Once he beat her so bad she had enough and took the kids and went to my parents house and she finally called the cops, but it was too late, a few days later he broke in and killed them all including his own kids and my sister and even my dad, at first my poor dad tried fighting him off but dad was in his late 60s and the bastard had a gun, he beat my dad and hit him in the head with it before he shot and killed him right infront of my sister, who he then sexually assaulted again before killing her as well, he emptied his gun in her body, after that he went and killed both of my nieces as well and he took his own worthless life after that.

My mom and younger sister were out shopping that day and thank god they were because the bastard would have killed them as well, this is the worst thing that I’ve ever experienced in my entire life and I genuinely don’t wish this on my worst enemy. My mom went crazy after that and now she cries herself to sleep every single night, she blames me for introducing him to my sister, she’s struggling and I don’t wanna fight with her and put even more stress on her but I told them when they started dating that I wasn’t fine with it and they all said it wasn’t my business.

But I’m wondering, like of course I wish I never brought that bastard home or was ever friends with him but you can’t change the past, but is this whole thing my fault?