Thank you. For me, I've really only seen this as a recurrent thing for sure with hotheaded Sims. One of my Sims had this problem just as badly before he was a Vampire as after. And no, these are not Vampiric Spars; it's just plain fighting. And again I should stress that at first I played him as a human and even then, he had this problem. I added a screenshot I made today where he can be seen autonomously fighting Caleb, whom he has an incredibly high relationship with, completely out of the blue (here he is actually a vampire). He was happy, Caleb was happy and they were having a casual discussion when it happened. There was no provocation/instigation at all. After this their friendship bar took an extremely big hit, as now it's only barely filled at all while as you could see, it was filled to the top before the fight happened. You may understand that it's kind of annoying to be forced to monitor my Sim all the time to make sure this doesn't happen again.
Having said that, I do like a bit of a challenge in my game and so I wouldn't mind the fact that my in every way dysfunctional Sim would start random fights with people. I did choose traits like hotheaded for a reason. But these people should be strangers or despised sims or enemies, not best friends and lovers. Actually, I wouldn't even mind it all that much if he fought loved ones, even, granted that he has a mood that correlates with his aggression, like Angry or Tense. Then it'd at least be kind of understandable why he'd suddenly go ballistic. :D
My situation is really similar. I have a male Sim who was created as Young Adult vampire, so he has no traits based on parenthood values (but I have this pack). However, he does have hot headed trait. He fights with his sister and brother-in-law (both maxed positive relationship) on a daily basis, usually with no negative moodlets. Sometimes they aren't even talking at the moment or they're in different rooms, but my fighter randomly decides it's time for some family violence.
Just as @Fierande said, I'd be fine with autnomous fighting, just not with loved ones and/or when there's no indication for that as negative moodlets.
It sounds like your issue is Vampiric spar which is normal and have no negative effects.
October 2017 - last edited October 2017
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I'm having this happen in my vampire family in Oasis Springs. I just have base game, Holiday, and Vampires. The sim doing it does NOT have a hot-headed trait. He doesn't have any negative traits. He's not mean or evil or anything. He will randomly start fights with his son or grandsons when both are very happy, and take their relationships from half-full green to half-full red in one interaction. He's been alive a long time and has high skills and such, but I still don't see why/how he can do it in a good relationship and mood, under circumstances when the game wouldn't let me select a vampiric fight manually. It takes a long time to build relationships back up when this happens.
We've had trouble reproducing this issue, could someone possibly share a save file where they've seen this? Thanks much for the help! https://answers.ea.com/t5/Bug-Reports/INFORMATION-How-to-Send-User-Files-to-The-Sims-4-Team/m-p/6065...