Source?
Very difficult to get data on people killed by illegal immigrants. But the numbers on people killed by rifles (not the subset of AR15s) is obtainable and about 340 a year from 2007 to 2017. Given that this is all rifles, the number for AR15s is probably less than 340. So if you believe that the number of people killed by illegal aliens is over 340 a year, and this is not such a stretch as one might have thought, then the claim is correct. If you do not believe it, then it is not. But I've not been able to find reliable data on it.