It's either, say Roose was poisoned or die .Ramsey may even kill him just in case. Who's he gonna run to?

As far as the trust thing, I wouldn't say Roose trusted Ramsey. He just always felt that power over him. He knew how psycho is son was but figured he'd never dare cross that line. The way he constantly threw those little jabs at Ramsey he felt comfortable being the only person Ramsey was afraid of.