Are “most” police shootings unjustified? No, they are not.
I'd say that the prevalence of questionable shootings these days makes that issue a lot more murky than it used to be. There's a definite "shoot first, ask questions later" mentality that cops in this country have adopted in the name of "officer safety" and it's led to more guns being used in circumstances where they probably aren't justified or necessary - e.g. conducting a SWAT raid on a small-time marijuana user, or "disarming" an angry granny with a kitchen knife.
I still remember the case out here in LA a few years back where the LAPD (a female officer and male partner) gunned down some old homeless lady because she "came after them with a screwdriver."