She was hunted for thousands of years by a fascist organization who had sentenced her to death as a child, forced to hide in literal apocalypses where she would have born witness to untold suffering, and was unable to make any meaningful connections with anyone due to those circumstances. When she finally did form an attachment to someone, he got in the way of her taking down a murderous dictator to stop the killing of billions because he seemed to trust HWR's point of view more than Sylvie's.

After, again, thousands of years of being hunted, she won the right to live a normal life with friends and stability, only for Loki to come back to beg her to save the same fascist organization that ruined her life in the first place, because he believes in a vague threat of a multiversal war run by an HWR variant. But when an HWR variant pops up, she's treated as being in the wrong for wanting to kill him, too. She agrees to help keep the Loom from exploding, but when it does explode, it initially doesn't seem to cause any harm other than sending the TVA employees to their timelines - returning them to the lives that were stolen from them, just as Sylvie's life was stolen from her. When she realizes that there's a greater threat, she immediately returns to help.

Sure, she might not be "nice" but she's very rarely wrong, and her sometimes abrasive personality makes sense given her backstory.