Looking at this from another angle - there is nothing at this stage to say that the attacks on the elderly people aren't connected. Many serial killers/rapists target weaker/more vulnerable victims when they start out. Who's to say the old people weren't 'practise runs' for other crimes the killer wasn't yet bold enough to commit?
Chaim's murderer was bold as brass, entering a populated building as he did. But he still attacked a (most likely) sleeping 15-yo boy blitz-style, aiming for the head (instant control) and wasn't very sophisticated about it. Maybe he he'd had enough practise to feel confident in some ways, but wasn't yet very confident with handling live victims. So he targets the elderly/kids..
I cannot shake the feeling he was familiar with Chaim and his habits in some way, however.
I thought the two previous attacks were somehow connected. Could well have been a killer who was beginning his killing arc, and so chose weaker,more vulnerable targets. I could not find anything to indicate that the two elderly people were robbed. Just that the killer had entered their homes at night, and probably they were killed while sleeping.
If these three murders were all connected, it could have been either a person who had been in all three locations for a specific reason, i.e. to do some work, or, it could have been someone from the yeshiva, who just wanted to kill. As a gambling woman,my money is on #1...jmo