Sure kicking in a door by itself isn't such a big deal... but being able to kick in a door, murder 2 adults and take another hostage is a different story. We're talking ex-military, avid hunter or deranged schizophrenic stalker stuff with all that combined.
And unless you knew the family extremely well, you could be walking into a really dangerous situation. Just imagine being a total stranger to the area, randomly picking a house out in the countryside (which is typically full of hunters) and trying to pull something like that. I'm a grown man and I'd vomit myself sitting in the car just trying to work up the nerve, not just because it's such a horrific thing to do but also knowing the next few minutes of my life could easily be my last unless it goes 100% totally according to plan. How's he to know that they didn't see him walk up the driveway and have a shotgun waiting for him? It takes a special type of nutcase or highly motivated, obsessive stalker to even attempt something like that.
So it's not about being strong enough to kick a door in... if we're talking about a small woman who pulled this off..... that would be shocking to say the least. What's the motive? What would motivate a woman enough to do something that? I get your point that it's possible for just about anyone to kick a door in but that is basically arguing semantics when you add up everything else which doesn't accomplish much.