This is why i'm actually gutted Rob is the killer. Especially after tonight - out of all the suspects, including Tracy, I never thought it would be him. It seems so random for it to be him, and he really didn't have that much of a motive - maybe if he was getting married to somebody who would break up with him if he was arrested for pushing stolen goods, but his wife to be got out of prison for murder.
To be honest, it reminds me of them turning Karl into a psycho - the writers didn't know what to do with him so they turned him evil. Tonight's events were shocking, but mostly because I honestly don't think it is something Rob would do.