Considering that the asshole tried to kill Sherlock thinking he was his brother, Peter doesn't find it disturbing at all.

"It'd surprise me more if it was," he says bluntly. "Whoever hired him sure as hell isn't worried about being discreet."

And it would be tidier to get rid of them both, especially since they're the only Starks left since their parents' demise. Which Peter is suddenly finding to be terribly convenient.