You can cut the valve off the PEX line. At various big box stores you can get sharkbite or other push to connect valves that will go right onto PEX so you don't have to use any PEX crimping tools. the push to connect are removable so you can replace it if that one breaks someday. Since it is under the sink and exposed, you will be able to see if there are any problems with the fitting. Some people do not like to enclose push to connect fittings inside walls.

I personally dislike PEX and push to connect. I would really prefer copper lines and good olde solder and threaded connections. But progress happens.