IT also used Henry, I believe, as a fail safe plan. If IT isn't able to defeat them on IT's own, Henry will finish the job.
I don't think this is entirely true. When they were kids, It planned on killing them all, Bowers and his crew as well. It used Bowers as a convenient scapegoat after the fact. Henry would have died if Belch hadn't stepped in and gotten it while Henry ran. As adults, It wanted Henry back for fun and games and to set things RIGHT. Henry, just like the rest, are unfinished business. Did It put Bowers on the path to kill them as adults? Sure. But you have to remember that It was playing with Henry as a child too, and this was all a resonate echo. I think the creature wanted Henry there because Henry had been there before. It didn't really think it needed Henry as a backup plan. He was always part of the plan. The creature believes itself to be an immortal God-like entity, with only the Turtle like itself (somewhat). Failure isn't really an option that crosses its mind. You have to remember that It called them back. It made SURE that Mike made those calls. It worked very hard to get the Losers back, just as it went out of its way to go get Henry. Think of it as some weird OCD, i.e. the monster wants reality set right to reassure itself. When Henry failed, it got itself ANOTHER Henry to make due.
What I do agree with is that It set Henry on them with the clear intent to bleed them the way a Bullfighter might a bull. It wanted to make sure their union was further broken, by having a couple of them dead or at least incapacitated. Could it have done this itself? Sure. But that isn't really the monster's goal. It wants REVENGE as well as putting things back the way they should be. A quick brutal Bowers-death for the lot would not be satisfying. Henry was a tool to disrupt and savage them. The monster wanted the Losers broken as a group so it could make them pay as individuals. More to the point, It knew all to well that there would be another showdown in sewers. The point was set the board up as it was before and replay the game exactly with this time getting the correct (from the monster's point of view) outcome.
I don't think Penny can make someone evil natured, IT can just add fuel to the fire. Influence and advise is provided, not possession. IT helped Henry out of the Institue in the later years, gave him a ride, gave him the knife. Whispered sweet nothing's in his ear and if Henry questioned IT, IT put him in his place. Like the part in the Christine car with Belch, I think. I forget the whole conversation but I remember Belch/Penny got up in Henry's face or gave him a death glare to let him know who's really boss, and could've killed him years ago.
Well it can ride dogsbodies when it wants too, particularly those with weak minds. The more foul the person, the easier the way in. It slides in to the inner darkness of a person quite comfortably. It doesn't have to influence people or whisper sweet nothings. So the question is why does it do in some cases and not others? I think it likes to play with its food and setting chaos in motion takes less energy and effort. In the end, we know that It is female or at least it has taken on a female aspect in reflecting our reality to reproduce spawn. This long pregnancy clearly takes up a great deal of its energy too and has for a long, long time. Every bit of raw power it applies to reality takes it away from that goal. In short, I don't think it seeks to corrupt humans or really cares much about them beyond food and fun. I just think it is lazy and/or frugal with its energy and time. It plots and plans only when it must. Most of the time we are out of sight and out of its mind. Our notions of time mean nothing to it. Morality means nothing to it. If/when it manipulates people indirectly with gifts, nudges, commands, or threats... it does so to conserve energy... not because it can't do the thing itself.