Personally,

I LIKE the Prince, and a lot of you would too if you actually considered the fact that because of the alternate timelines and the subsequent alternate reality, he is NOT THE SAME PERSON as the cruel version of himself and never will be. That, and the fact that her other options are inadequate and the author knows that.

Allendis is a creep, and that's a fact. He's been creepy and manipulative from the very moment he's met Tia, and I don't do yandere second male leads (they tend to try to kill everybody, FL included—don't act like, if it came down to it, he wouldn't try to kill Tia's father if he got in the way). He creepy, manipulative, and obsessive to the point of ridiculousness.

And Carsein, though i love the fiery red head, is too immature. The author placed him there to lessen her dependency on Allendis, and I enjoy that. Because he is so immature, she couldn't help but see him in a brotherly way. And while he matures as he ages, he is still way less mature than every other character his age in this manga. They simply were never meant to be anything more than friends with a close sibling relationship.

I like the prince. I like his character design, i like that he has experienced some growth and got to be more open about his emotions (slowly but surely) and, more importantly, he's the only reasonable option out of the two extra male leads that the author created.

Now, if there was another character who was better than all three of them, I would be down: say, a cool and level headed type character, who was interested in her but not in a yandere way, and had the power to upsurp the throne (like a second prince) then I would be down. But, that won't be happening and, out of all the options the author has presented us, I LIKE the prince.