Read this in the 70's but (somehow) felt it was a bit older.
Protagonist driving late one night and this alien thing with tentacles attacks so he kills it with his pick-up.
Next day police and authorities are at his house looking for him but his wife hasn't seen him. He was witnessed dumping dead alien outside the small town police station. They ask if he mentioned this when he got home and she's all "that impossible man!" Because he hadn't even told her.
The story progresses with sightings of him around the vicinity every few hours, with dead aliens killed in a variety of ways. Experts realise the creature is sending fresh versions of itself with countermeasures to it's previous deaths but the protagonist is always ready with a new means of killing it.
Army intelligence have his records and know he is capable of something called King Marine Syndrome, such people, under the right stimuli, can win major battles.
They anticipate the next alien appearance and set things up. He is in a small boat (I think) crossing to a forest alien craft site. Just before he reaches it the high command shell the area in a long salvo, this does no good to his stealthy approach and induces in him the feeling that high brass always messes up and he is on his own. King Marine mindset emerges!
He gets to alien craft and it emerges for another round. This time he instantly drops his weapon and (kneels?) concedes.
This action changes alien intentions for future hostilities - IIRC