I'm quite certain that this claim is total bulls**t. And I'll bet that the proportion of "killers" among illegals is about the same as that among legals.
IIRC, I read somewhere that the number is incorrect as it was based on a study that said like 25 percent of the people in FEDERAL prison for murder are non-US citizens. This percentage was then applied to the general population of US prison, and further applied to take all "non-US citizens" as illegal aliens, which is not necessarily the case.
IIRC, the percentage of non-citizens in the US (local) prison population is substantially lower... that's not even counting only the illegal immigrants. I'll have to look it up, but I don't have time right now.
I'll take that bet, since I know that there are no chapters of M-13 among legals...