Danno was charged with first degree murder, yet first degree murder requires specific intent to kill with premeditation and deliberation. In this case, Danny shot at a door to break the lock, which does not demonstrate premeditation or deliberation. The most he should be charged with is second degree murder, but most likely a negligent homicide (accidental) killing.
The close-up of Thad's gun when Ann picks it up from the floor shows it to be a short barrel S&W .38 with blue finish, the exact type carried by Five-0. When the gun is found by Five-0 and identified as the one wielded by Thad, it is a .32 with bright finish.
In the opening foot chase, a car blocks Danny's way, so he adroitly jumps over the hood. But if you watch, you can see he easily could have gone around the car and actually saved time. He (or his stuntman more likely) had to slow down and time his footing to get to the car at exactly the right moment to make the jump.