I agree with you. Osimhen is not a footballer with an extremely bad behavior. He is a good professional. After the Bologna game where Osimhen missed a penalty, the club's official social media account posted a video mocking Osimhen. I think that's the knot and the starting point of the problems. Osimhen was offended by that post and nothing was ever the same again. The best thing for Osimhen is not to play for Napoli anymore. I'm sure he will be very happy at Galatasaray and he will give a unique performance. And after one season at Galatasaray, he can play for Chelsea or Paris Saint Germain.
Osimhen behavior is still unstable because of his young age, he actually doesn't want to leave Napoli so quickly because he thinks the club has been instrumental in improving his career. Perhaps the root of the problem lies in the video uploaded from the club's official social media account, this action will offend anyone because penalty misses are commonplace.
Now that the relationship between the two has broken down and there is very little chance of reconciliation, the refusal to sign a new contract proposed by the club indicates that Osimhen also does not want to continue his career at Napoli. Now that he is at a different club on loan, he must be able to show his best performance while defending Galatasaray to attract the interest of other clubs to use his services.