That would only serve to produce sympathy for him. Taking out a high-profile leftist would be needed (Bernie, Fauxcahontas, et al) to gin up support for Hillary.
Look at 1963/64. JFK knew he was going to have trouble facing Goldwater, when it would’ve been a real contest before Dallas (that’s why he was in Texas a year before the reelection, trying to mend bridges in a state that was hostile to him and his liberal agenda). Martyring JFK assured LBJ’s landslide win.
The killing of Jo Cox will only result in some other Islamist apologist filling her seat. 3 weeks from now no one will give a rip about her. The murder of Trump however will leave open a spot for a leftist bootlicker like Kasich, Romney or Ryan to ride to the “rescue” of the “conservative” movement - i.e - an Islamist/ MB / ISIS third term in a Hillary Clinton residency
Lest no one dare speaketh ill of dear Saint Jack...