If he removed it after the murders it is obvious, he would no longer be visiting and scared Jeremy would steal it.
My mom's cousin left his tools at the house his mother shared with her brothers (all were elderly so he fixed up things for them). After she died he still kept the tools there to take care of his uncles. Another cousin did some illicit things and sold the house to keep the money for himself and either stole the tools or threw them away claiming they were not found by him.
If he did so before the murders there are still logical reasons f or it but it is less easy to know for sure why
I'm sure it's perfectly obvious to you, as a prosecutor, that if he removed it after the murders, he just walked in, removed said fire arm and walked out again ,without informing anyone of what he'd done, without it looking in the least bit suspicious. I, on the other hand feel such an action is rather odd, not to say, unfeeling, given that his relatives had died so horribly.