There's really no reason for Freeza to betray his own universe. It really doesn't benefit him in any way.
If he wins, he gets to use the Super Dragon Balls and can make himself alive/immortal/more powerful than Zen. A surprise attack on U7 makes
some sense from that perspective i.e. take advantage of the chaos of the situation and eliminate everyone he knows is a threat from his universe in order to get what he wants. It's not like Frieza hasn't resorted to tactics like that in the past and he's cocky enough to think it'll work because he (thinks he)'s supposed to be the strongest in his universe.
Also, it'd keep up the running...gag(?) that Universe 7 keeps letting their guard down against threats they shouldn't be. Frieza shouldn't be trusted to play nice given how he's...y'know...he's Frieza.
EDIT: I should point out I haven't watched the episode so please don't RE: any spoilers, thanks.