Re: tampering with evidence. Get that out of your head. Rule number one for post shooting is "Security" - which at this point means ensuring the safety and security of yourself and others around you by making sure the suspect / assailant is disarmed / incapacitated and no longer a threat. Worry about preserving the scene after we ensure everyone is safe.
Kicking the gun away is not theoptimal choice if you have help, better to secure it, but if you are by yourself it's an option,