How so? Our Starfleet heroes shoot bad guys dead left and right. The feeble excuse of "self-defense" generally does not apply, in a world of stun guns and general Starfleet superiority in numbers and all.
It's their paid day job to eliminate enemies of the state. Why this should exclude UFP citizens is unclear, or conversely it's unclear why the right to trial should not be extended to Klingons or Salt Vampires. But "taking a villain alive" isn't how Starfleet gets to define victory in the general case: many opponents are too slippery for that.
We have still to meet a police force other than Starfleet in the UFP. Or a fighting force other than Starfleet. Yet interestingly enough, we have still to witness a trial that would not be internal to Starfleet, too. TOS started out by establishing that civil crime gets you a stall at the funny farm, and never a punishment, leaving Starfleet the only organization to still practice keelhaulings and the like. TNG expanded on that by having the villains be superbeings or perhaps hypobeings, unsuited for judging or punishment. And both showed Starfleet dishing out field judgement and executions. The rest of Trek sort of has to live in the shadow of that precedent, it now being damned hard to come up with a civil court scene. (Within the UFP, that is; alien courts can still flaunt interesting aspects of Earth's very own culture of judgement, so we're not left completely high and dry.)
Timo Saloniemi