1. The Rogues hold back a little on Flash, they hold back a little on him. No one wants to escalate things to the point his friends like Wally and Jay start dumping people into the speedforce because they sub-zero fatalitied Barry.
2. Johnny isn't equal to the Flash. Barry has a natural connection to the speed force. Johnny has to keep injecting himself with drugs to maintain his connection and it weakens over time. Being a junkie Johnny is also an impulsive, cocky idiot-even more so than Flash usually is.
3. He didn't know how dangerous Cold is for a speedster to fight and thought he was helpless when he took the gun away.