Trigorin is a famous writer, and the lover of both Arkadina and Nina. His whole personality circles outward from this core identity of "writer." The need to write and to be read has defined his whole life. It's the reason he's never really been in love. "When I was young I never had the time," he tells Arkadina in Act 3, "I was always trying to get myself published, make a living" (3.96).
Trigorin has a long, long monologue in the middle of Act 2 in which he describes, to the rapt Nina, what it means to write. There's a lot of Chekhov in this speech, including a compulsive need to record the events of life:
"Every word you and I are saying right now, every sentence, I capture and lock up in the back of my brain. Because someday I can use them!" (2.100)
(See "Writing Style" to learn more about how this applied to Chekhov.) So Trigorin is excited to talk with Nina because the young women in his stories don't ring true. He asks Konstantin whether the outdoor stage still exists, because he wants to put it in a story. Trigorin is a creative cannibal, devouring the people around him and putting them through the sausage-grinder of his art…
…which totally makes him a great match for Arkadina. It's a partnership of convenience. They offer each other sex, reinforced fame and success, and just enough attention but not too much. Arkadina understands her role in the partnership very well; just check the Maupassant quote she cleverly evades reading under "Shout Outs."
When Trigorin's head spins with attraction to Nina, Arkadina wins him back with bald-faced flattery. He's easy to convince. "I haven't got any willpower," he says. "I never have had. I'm a limp washrag, always do what I'm told" (2.104).
Trigorin just doesn't love people. He loves writing. And Chekhov doesn't judge him for that. We might disapprove of his callous treatment of Nina—Konstantin certainly does—but in Act 4 he's not a villain, just the slightly distracted writer under Arkadina's thumb. Trigorin longs to fish. He's forgotten about the seagull he ordered stuffed in memory of Nina, and he's forgotten Nina. But he has probably put her in a story.
So is he a bad dude, or just someone under the spell of writerhood? That's up to you, oh wise Shmooper, to decide.