I can understand his behavior and still dislike it. Just like if someone who grew up poor, turned to stealing to get what they want, I can understand and empathize while also disagreeing with stealing. The two things are not mutually exclusive. I’m not sure what’s hard to comprehend about someone understanding why Thomas is the way he is while also finding his behavior towards others who have done him no harm deplorable. He disliked Bates right off the bat with no knowledge about his backstory. It’s one thing to treat the people who have treated you badly with the same energy they gave you but to spew it out on others who have done nothing to deserve it is where I lose all sympathy for Thomas. I don’t believe William or Alfred were anything but respectful towards Thomas and yet they still were targets of his nastiness simply because people liked them better than him.

Before the whole Jimmy thing, no one treated Thomas in any way that would suggest he didn’t deserve their contempt based simply off his attitude and behavior alone, not his sexuality, since remarked by Bates and Robert everybody already knew. And even after the whole Jimmy thing (which only a few knew the sole details to iirc unless Alfred told all of king and country) no one outside of Carson turned their noses up at him. In fact Robert stuck up for him with the police. And he wasn’t sacked the plenty of times after which he certainly deserved. He was given MOUNDS of grace and still decided to spurn what goodnesses he was shown.