I often think about killing people. Mostly it's random.........as i have OCD, i've always had to deal with vivid intrusive thoughts and images.
In the past, violent images (of myself attacking/killing others) was very disturbing to me. Now, not so much...though there are times it can be.
As far as the "revenge" component you mention.........yes. That is there too. I've fantasised about killing everyone who has left me. I still wouldn't rule it out

............but yeah, i think a fair percentage of borderlines have homicidal thoughts. Especially ones, exposed to violence.
I think the important thing, is perhaps to figure out what it is that you get from this fantasy. (why do you like it?) and what it is that scares you about it? (for me that would be going to jail, but perhaps you fear something else that speaks to your "character" )
I wouldn't be overly afraid of this though.........the fact you are aware suggests you aren't likely to be overly compelled to do anything that could land you in prison or in a mental health facility for a lengthy period of time.
There are much bigger issues than this, that this speaks to...........learning to deal (or accept) those....WILL alleviate the need for this fantasy, while at the same time.....perhaps alleviating any guilt you have over having it.