No. There are other ways to resolve conflict than hitting it over the head repeatedly.
Also, in the words of Qui Gonn, "There's always a bigger fish." If the Ranger can ginsu an enemy, he should expect there are creatures that can do the same back to him - and probably worse. (I also have to ask, was the Ranger taking this in melee solo, instead of with a friend who might be able to absorb half the attacks or so?).
He won initiative and so was the first to act in the encounter, before the enemies. He had no way of knowing how dangerous the weretiger was (as there were other opponents). If he waits for his allies, then he's ceding the advantage initiative gives him, and in fact, if he hadn't rushed the weretiger, the weretiger has a 40' speed and a pounce maneuver that they can use if they move 10' before attacking, so whoever ended up being targeted would have been in a worse situation (most likely the Bard or Sorcerer). Or alternately, if you read their multiattack again, the weretiger could have just as easily used their longbow attack four times at any target they wished to if no one had closed to melee.
Could he have readied an action? Sure, but at the cost of two of his own attacks. Basically, most of the ways he could have avoided the beatdown would have run counter to the character he wanted to play (a dual wielding skirmisher) and would have likely opened up someone else to that same beatdown.
If his luck had been a little better he wouldn't have gone to 0 outright, but the fact is, he did, and I don't think "moving to attack in melee" should be considered something nobody should ever do- because what then? I'm reminded of a 4e encounter I had once with a dracolich. Our Defender moved to attack and mark the enemy, hoping to disincentivize it from attacking freely. As he entered it's reach, he got by a reaction tail attack that stunned him, ending his turn on the spot.
There was little way for him to have known that the creature could do that (sure, his allies can roll monster knowledge checks and maybe find out, but the DM was fairly strict and only allowed them after you actually observe a monster do something- a not uncommon scenario, and likely the main reason why 5e doesn't have dedicated monster knowledge rules- at least, that I know of).
I'm not saying that there isn't a school of thought that says the first turn of every battle should be to back up, dodge, dive for cover, what have you, but there's a counter for every such strategy- you want to bunch up to benefit from your Paladin's aura? Get fireballed. You want to dive behind cover? The enemy can do the same, or Hide, or maybe they're a rushdown monster that's faster than you and is armed with the special senses needed to find you (said weretiger's ability to pinpoint the location of characters within 60' just being one example).
And there are enemy monsters like archers and casters that you very much want to close with and pin to the ground. When they later fought the Sorcerer, they
were, in fact, more cautious, and that just resulted them in being caught in a sleet storm. Different fights require different tactics, and that requires some way to know what your opponents are going to do. Unless you're somehow precognitive, that means that your scissors will inevitably encounter the enemy's rock, and saying "well, you could have chosen paper" without knowing there was a rock is incredibly unfair.
Are there parties who insist on slowly creeping their way forward during adventures, using stealth as much as possible? Sure! But there's a risk there too, of one straggler getting away to set the whole dungeon on alert. Or running into a monster with a special sense that foils stealth. Some adventures are on clocks, and you can't take that kind of time. The list goes on.
If a DM wants to reinforce certain kinds of play, like "don't run into melee", that's fine. But if you're not setting out to do that, then I don't think the system should be punishing you for it. OR, alternately, if the system does this it should tell the players straight up what is expected of them. "Enter melee and drop to 0" is a pretty harsh lesson. And if the consequence for dropping to 0 is instant death, well, that character didn't learn anything.
I've had similar discussions in threads about penalizing people for dropping to 0, like taking levels of exhaustion, and while that might work fine for some, all I've ever seen out of similar things (like wound penalties or "death spiral" mechanics) is it makes nobody want to enter melee, because they can go from high hit points to 0 between one turn and the next without being able to do a thing about it.
And given that there are entire classes devoted to this concept (looking at you, Barbarian), I think it's disingenuous to disincentivize someone from being a member of said class. That said, I know the dissenting crowd is already ready lining up comments such as "play smarter not harder" and "git gud", lol.
As for me? I'm going to be a lot more careful with my encounter design going forward.
