One thing to bear in mind; some crypts are inherently dangerous due to layout and RNG spawner presence as well as mob type and rarity. It is quite possible to have an entrance with multiple open paths each containing one or more spawners. It is also possible that any blind tunnels may contain rare mobs and/or spawner such that it is a death trap to most swamp level players.

Bear in mind you do *not* have to clear any dungeon entirely. There are plenty of them in the world (barring large population multi-player) so mark the dangerous ones for 'do later or never' and move on. This is also true in other biomes, but in my experience Swamp is the worst for generating possible death traps that are, for a solo player, practically impossible to do a corpse retrieval from.