Should I replace a kinked hot water supply pipe?
The copper pipe is creased enough that if the shut-off valve is twisted to the 'on' and than 'off' position several times the force exerted will probably break the already weakened pipe. It would be wise to repair the damaged pipe now rather than repairing it later (and possibly dealing with a flood clean-up). As Aloysius commented about using a no sweat replacement valve will make for a simple repair. Use a pipe cutter for a smooth even cut. You will need to leave at least a 1-2 inch stub out from the wall to have enough purchase for the new valve (looking at the photo you should have plenty). Don't forget to ream the inside of the copper after it's cut to remove burrs. FYI: 1/4 turn valves are easier to use and last longer than the rubber washer "screw" type valves.