I have already explained it. This isn't about hindsight. It called common sense. Gohan could have killed Buu the moment he showed up again. He didn't have to listened to Buu or even humor what he was up to. Nothing Gohan did was out of caution since he should have attacked the moment Buu started to rile up Goten and Trunks to fuse, which should have been a big red flag that they shouldn't let the fusion happen.Nejishiki wrote: I'm not quite sure where you're going with that. As the scene was presented, everyone was anticipating Boo to do "something". It was out of left field for him to suddenly shout at the children like that with tensions as high as they were. We know in hindsight that it was safe to attack Boo. Yet moments before that, he escaped & returned confident with no apparent changes but with an obvious plan concocted on his demeanor. Something strange was afoot. The opposing party was attempting to gauge what he was planning to do before attacking too early.
In their perspective, a trap was at play & they were right. They just happened to fall for it anyway despite their fear of the new unknown. With nobody, especially Piccolo, disagreeing with that level of concern, their actions were reasonable. Reasonable minds, but not infallible minds. What made everything fall apart, as per Boo's intentions, was Goten & Trunks jumping in without concern if their mettle was called into question.
I feel that questions such as "why didn't he smash half of his brother" are weird & not minding the current mindsets of everyone involved. Of course we're aware of better options. In-universe, in real time, it happened too fast for anyone to sit & reflect on each panel individually (which they already did beforehand). Rather than viewing it as being written off, consider the heat of the moment for the characters. You reference Gohan's past. He didn't think long term about consequences with Cell. He, along with the rest of the group, considered the long term consequences here. Together, it was concluded Boo wasn't planning to destroy Earth thus a repeat of Cell's explosion wasn't on either Piccolo or Gohan's mind & that's plenty reasonable, I feel.
This is just a situation where everyone held the Idiot Ball except Buu, which is why he came out ahead.