If he did put up a big fight then where are the marks on Sheila or Jeremy? Just because you do not personally believe something does not make it untrue. There are many people who think otherwise. The hard evidence is (and this is what we must go by) is that there were no marks either on Sheila, or Jeremy which were consistent with a "big fight". Logic: He did not put up a big fight. Conclusion: He was already incapacitated. Pure logic my friend.
He was shot 5 times in the torso upstairs & on the stairs. But was found in the kitchen.
So he had the strenght to get to the kitchen where the pathologist said he put up a big fight for life.
The likelihood is Neville fought in the kitchen with 5 torso bullet wounds.
Surely with those injuries he could still have won a fight with Sheila. Or at least left marks on her. However a young, fit Jeremy is much more likely to carry out a successful beating without getting injured himself.