Not only can you not spell, don't say widely untrue statements. In most jurisdictions, you need to be under active threat to shoot someone. So unless had his gun holstered and able to pull it while getting punched in the face, theres no legal right to shoot since the OP would be walking away and would thereby no longer constitute a threat.

If he pulls it after getting punch and then shoots him in the back- bam, easy murder or manslaughter case and say hello to life to 20-30 in prison

Follow the guys saying get a good lawyer, and do what they say. Likely with a good lawyer you have a good chance unless theres any video evidence or cameras at the park that caught the incident.