No, you did nothing wrong.

There are a few things here that stand out to me regarding the Glassstaff fight on which I can elaborate a bit.

Surprise is very powerful

Average damage and healing numbers compared to average hit point pool sizes just mean that the most effective way to win a fight is simply to put out more damage faster than the opposing party. Surprise helps a lot with this.

Smite + Crit = Ouch

Paladins are notorious for their crit smites. It is indeed correct that a crit will also double the damage dice of a smite, so when it happens there's not much you can do but smile, be happy that your paladin got their moment in the spotlight, and say goodbye to your monster. This is a big part of why some people play paladins and it does use quite a bit of their daily resources.

Low level encounters are very swingy

Low level encounters are in my experience quite hard to balance, simply because your players' resources are still very limited; their hit point pools are very small, so a single (un)lucky roll from a monster can knock them out. They have few spell slots to mitigate incoming dangers. They have only one attack per turn and low attack bonuses, so a little bit of bad luck means the whole party might put out no damage at all for a whole round. This means that you have to be very careful with setting your monsters' hit point maxima and average damage outputs, or a fight can turn very sour very quickly - at low levels at least. This also means that it's generally better (depending on your group's desired playstyle of course) to make encounters a bit easier than normal.

The action economy

5e takes great care to make sure that the action economy is kept stable, that means creatures (monsters, npcs and players alike) must have a very good reason to be able to make more than one action per turn (with the exception of legendary and lair actions for higher level boss monsters, which only exist to fix exactly the problem in this case). This, again in combination with the way the combat works (more damage faster wins), means that a group of four versus a single enemy has a huge advantage, because the single enemy has only a quarter of the actions available and is a lot more vulnerable to bad luck.

What you could do differently next time

Make it harder to surprise the enemy

Maybe the wizard had cast alarm on the other side of the secret door, so he knows someone is there, or make the secret door harder to find.

Dynamically scale up a monster's max hit points

This is something I do a lot. My players usually play pretty optimized, so if it looks like an encounter is about to face an abrupt and anticlimactic ending, I simply retcon a monster's hit point maximum to a higher value that is still within their hit dice range. So for example "Glassstaff", who uses the statblock of an evil mage has 5d8 hit points, or 22 on average. In this case I might have just increased his maximum to the actual maximum, 40 (5 * 8), which would have left him with 9 hit points after an average critical smite, so he could at least act once.

Balance the number of opponents

After all, that's what the guard quarters two doors further down is meant for. Either have a few guards in the room already, delivering a report or whatever to their boss, or have them join the combat mid-fight, because they might have heard a noise. Although, if your players made sure to prevent that scenario, either by clearing out nearby guard quarters or, eg., casting silence to muffle the combat noise, make sure to give them the advantage from such careful preparation.