That sounds like the kind of post where someone would say, "Why didn't they just shoot the gun out of his hand?"
Tell you what, we'll give you night vision goggles and tranquilizer darts to go into thick brush to hunt aggressive lions and tigers and bears. Even if you managed to pop one with a dart, you'd be dead before the drug knocked him out.
The cops did what they had to do to secure the area.
A big can of tuna fish mixed with Valium?
Darts are not always reliable or effective. An animal that has been shot by one may still have time to close with and kill a human. These cops were not animal professionals, and shouldn't have had to take the risk.
A few years ago, the Louisville Zoo "lost" one of their lions, and the cops were not taking any chances. They were out looking for him with some serious ordinance. It turned out that he'd just found a new hidy-hole inside the enclosure, but it was a tense few days. What was real exciting was how he was found; a cop and two zoo employees were in the enclosure, trying to figure out how he escaped, when the cop spotted him hiding under the rock. How'd you like to be face to face with a full grown African Lion and the only thing you have is your 9mm?