Responsible? No.

Able to stop the bloodshed? Very likely so, if he has the willpower. And they are trying to push on him to take concrete action. Abstaining from using UNSC veto is an easy lever he can pull as head of the Executive branch, as is using the ‘bully pulpit’ to criticize Israel’s actions and highlight the behavior of the extremists in the Likud coalition cabinet. Neither require congress or the courts, just the will to act