Hi Cyberempathlover,
spoiler
Isaac is a bit volatile. In Simon's route, he's still harboring a bitter attitude towards the MC, as they ran away from the castle during the raid. (In his flashbacks, he blamed the MC for both his fortunes and misfortunes) so when the MC acts too blatantly in doing things for Simon, he's angry, jealous and vindictively kills them. If they manage to hide their alliance with Simon enough for him to convince himself of a more pleasant story between them for a little bit, he's satisifed. (Isaac is aware it's self delusion) At the end of his delusion, he gets all wet sad cat and decides he'd rather accept the MC doesn't like him, and just be put out of his misery by their hand rather than take out any more hopeless frustration on the MC.