A reasonable interpretation is that the officer was arguing that he had cause to fear for his life or that of other innocent people nearby, eg. his fellow officers.
He has said as much:
http://www.kansas.com/news/local/article208812519.html
Now, in this particular instance, sure, it's most appropriate to blame the person who set up the hoax to begin with, and the person who solicited his help. But US cops have done similar things in similar situations--SWAT assault or the like on the homes of innocent people--even when they haven't been set up by someone else. Were they not so twitchy--and so accustomed to being able to kill people with impunity--I believe the likelihood of Finch dying in this incident would've been much lower, hoax or no hoax.