Not totally sure if this comment is ironic or not but wanted to expand on this thought either way.
Using a reference with threatening language in it that is potentially unknown by the people recieving it, makes a death threat even more sinister as it feels like you're building in plausible deniability and trying to have your cake and eat it.
It's basically admitting "if I just said this it would cross a line, but if I quote it instead then it'll not cross a line for people who know it's a quote, but still cross the line for the people I'm threatening. So I get to make the threat and disclaim any intent at the same time.
And? Was he giving a recital of Tupac's lyrics / reading it out loud, or was it pulled out of that context and aimed at someone specific?
I can call your mother a hamster and that your father smelt of elderberries; just because it's a quote from Monty Python doesn't mean it wouldn't be insulting to you.