Separation of powers was built into our constitution to prevent which branch from consolidating too much control?

The separation of powers protects the people from the abuse of control from any of the three branches of the government - the answer is all three of them (the legislative, the judicial, and the executive). 

any of the three branches could have the power of sabotaging the work of the other two, so the power of the three branches has to be restricted - they need to be checked by the other two branches.