I think his dad was treading on thin ice before he grabbed Fred. His “father” says he will kill Wesley for the soul-stealing-stick and Wesley’s rather calm response was “I know. You raised me.” I think he was fully prepared for one of them to die in that exchange, and his “father” made the decision for him when he grabbed Fred.
Now, the relationship between Fred and Wes is complicated. And I have a lot of thoughts about it (because Wesley is my obsession. Literally).
First, she didn’t “know” how Wesley felt about her all along. Wesley was a complete chump when it came to her. At the beginning of Billy, he invited her over to his house for dinner, and then chickened out and invited Cordelia, Gunn, and Angel. Wesley never told her he liked her. He never indicated he wanted to date her. He never asked her out on a date. He did treat her like they were just good friends. Contrast that to how Gunn treated her. He said things like “Oh man, you know you’re beautiful.” He took her out to breakfast every morning because she liked pancakes. He complimented her and teased her and flirted with her. Why would she assume that Wesley is interested when Gunn is actively showing her his interest?
Then Wesley “betrays” Angel and the group and he is outcast. Fred and Gunn take their lead from Angel, and Angel makes it very clear that he will kill Wesley if he sees him again. Furthermore, Wesley doesn’t want anything to do with them. Did he just take Angel’s threat to heart or does he really not care anymore? It doesn’t matter. He’s on the outside. Gunn and Fred are left alone with Connor all summer, and Wesley is apparently watching them and monitoring Connor (since he knows what Connor did to Angel), but the actual threat to Wesley is removed. He could have contacted them any time. He didn’t. Of course, at that point, Gunn didn’t trust him because unlike Fred, he knew Wesley had a thing for Fred.
So, so far, we all know of his feelings for Fred, but Fred has no reason to believe he loves her. Then he starts sleeping with Lilah! And, worse than that, he implies very strongly that he was in love with Lilah.
In S4, during the whole Angelus panic, Wesley kisses Fred and makes it clear that he’s “not sorry” she and Gunn broke up. Fred seems intrigued by his response, and finally clued in. But the fact that he had lots of hot dirty wrong hot sex with Lilah is a major turn off. And not long after that revelation is made, Jasmine is born and they’re all put under a spell. This is not conducive to falling in love. Even if were, the timing is all wrong. You can’t hook up with your ex’s former best friend. And as hot as Wesley was in S4 (mmmmmmm) he did have lots of hot dirty wrong hot sex with the now-dead enemy.
And then everything is reset! Suddenly, the relationship between Fred and Wes is like it was around S3. She still thinks of him as a friend. He’s still got deep feelings for her. And Wesley’s insecurity is such that he will never, ever believe she (or anybody else for that matter) could love him. By S5 we know for 2 years that he loves her, and Fred is only coming to that conclusion because she doesn’t remember when he clued her in to his feelings before.
So considering the fact they were moving from “just good friends” to something more, I don’t think she was bitchy to him, or mean to him, or anything else. I think her actions were fully justified.
She did have a thing for Angel for a little while after Pylea, but I don’t think she was interested in Angel so much as the ideal. And she stated plainly that the reason she loved Gunn was because he couldn’t do something as horrible as revenge. The reason their relationship fell apart after that was because they both had to change how they viewed each other.