The Careers call Peeta "Lover Boy".
The Careers use Peeta to figure out what Katniss' skill was that got her the eleven from the Gamekeepers. The Careers also think that Peeta might know Katniss' whereabouts.
While in the Games, Peeta appears to align with the Careers.
In the beginning of the games, Peeta joins the careers in order to keep Katniss safe. This way her could lie to them about where he thinks she might be hiding and what her strengths/weaknesses are.
That Peeta has teamed up with the Careers and is helping them find her and kill her.
The careers
Sets them on fire
At the beginning of the games, Peeta joins the careers because he thinks it gives him a better chance at winning. He does quite well until the careers either see though him or finds out his plan, and injures him, leaving him for dead. Then Katniss finds him.
he is under a spell he cant help it
Their trust, and ultimately a little more time for Katniss.
Katniss and Peeta were going to break off the alliance the night the careers came to get them
They were using him to get to Katniss and then they were planning to kill him.
There are multiple reasons. One reason is that when Katniss broke open the forcefield, both the rebels and the Capitol came to pick up the tributes. The rebels didn't have enough time to find Peeta and get out safely, so the Capitol was able to capture Peeta. The other reason is President Snow wants to "toy" with Katniss by using Peeta as a sort of bait.