I hear the Traffic Alert start, which isn’t uncommon for aircraft 500 feet apart in this airspace, and as I look up Aircraft Y tells me he’s responding to an RA and his voice is clearly strained. I incorrectly said what I was thinking, which was “Aircraft X what are you doing! Climb immediately to 5,500” as I’m watching him descend through 5,200 feet right on top of Aircraft Y.He responds that he “thought they were on a collision course.” To which I informed him that his actions caused the collision course.