There's a difference between being careless and being in shock. We haven't seen their history, but Rick and Shane have been friends since high school and they're what? 30-35 years old? That's a long time to be friends and partners on the force. There's a strong, deep bond there. Knowing that Shane planned to kill him and having to put a knife in his gut and twist--a human being who has empathy and who feels would be traumatized by having to do that. Rick's still human. We the viewers are just accustomed to Star Trek style stoicism in male characters. Rick's reaction is more realistic. He's not an army special forces soldier trained to stay "frosty."