It does not make you a bad person if you only ship one pairing in the entire fandom.
I personally have written multiple pairings, but my true one is Harry/Eggsy. I know people who thrive on multi-ships, polyamory, one OTP, or none. (which, come on, why do I see only ten kudos on some fantastic gen works on A03??) All of those, to use the tumblr speak, are all valid.
As a disclaimer, I will never kill off Tilde or rewrite her into some massive bitch, but there’s nothing wrong with having them mutually split up or one of them deciding that the other isn’t the right person for them in the end. They’ve already canonically had a conflict about Eggsy’s job and what it means for their future, and I don’t think it just gets dropped after that, even after they marry. It’s a huge adjustment to being an average civilian to a super spy to a prince. If Eggsy has had no reservations or internal conflict about that, ever, I’m calling bullshit. If Tilde has never worried about their relationship while Eggsy was a spy or even how Eggsy is transitioning into a future ruler of Sweden, I’m calling bullshit, too.
Will I write Tilde leaving/Eggsy leaving/them mutually breaking up in order for Harry/Eggsy to happen? Yes. But it doesn’t make me a shitty person.
You can dislike a ship. You can wish it was better written or didn’t happen all together. You can, and please do, use tumblr savior or some other software to never see a certain pairing again. Control your fandom experience.
HOWEVER, we must be aware of the misogyny and implications when we make or consume art. This happens quite frequently in fandoms, usually with minorities. There are a lot of better posts/academic and news articles that can explain this better, but bottom line: no fridging or demonizing. Shit happens in relationships without having to resort to extremes.
Relationships are like this. People stay together. People drift apart. People discover that what they want or even what they need isn’t with that person. Even when wedding bells ring and decades of marriage happen, the problems are not over. No relationship is perfect and unproblematic—in real life or in fiction. No person is simply either an Unproblematic Ray of Sunshine and an Utter Garbage Trash Bag.
Let people write, draw, or consume what they want. It’s free anyway.