In It, by Stephen King, Henry Bowers and his two friends, Victor and Belch, follow the Losers into the sewers in 1958. Somewhere along the way, both Victor and Belch, which were in a trance, and were, like Henry, dogsbodies, get killed by their fears.
Why would It kill them? They could very much help Henry chase and kill the Losers.
The real question should be "why wouldn’t it kill them?".
It is known to kill indiscriminately and seems to be an opportunist when it comes to feeding. The Kitchener Ironworks explosion killed 88 children all at once. The fire at The Blackspot killed many army recruits and was motivated by existing racism - It only provided the "spark". It’s the same with the lumberjack murders and the Bradley gang shootout.
At the point the children enter the sewers in 1958, It is largely unchallenged. It doesn’t think it needs protection. Henry Bowers is helping to provide It with food so he hasn’t been killed, but it has already killed members of his gang: Patrick Hockstetter and Moose Sadler. Victor Criss can already see the extent of Henry's insanity so it’s possible but unlikely that It is protecting Henry. Nothing has hurt It before and Its monologues in the novel express surprise that these children have hurt it.
Basically It has no gooreason to pass up a tasty, terrified meal.
At this point since it's speculation, I guess I would say that Victor and Belch would have snapped out of their trances and tried to stop Henry. Don't forget Vic met with the Losers at Trackers. He was the one that realized Henry's deteriorating insanity, and he would have put a stop to the funny business. Or maybe It just wanted a tasty snack.
