WTF he's death!! Sewers SPOILER!

[Hunter of Shadows]

When i walked true the sewers and come at the part where i see a damaged gate but can't go true it yet i need to go upstairs to an dead end room, where rocks block the path.
In this room i see the Monster that was chasing me till the prison cut in half!!!
Why is he killed? I don't get it cause he was an evil monster but he got caught by something more evil then himself?
After that i didn't saw him anymore so i guess there was only 1 of him cause now the machine monster is after me who just needs 1 cut to make you die...damn just when i started to think these monsters where a laugh the real scary ones just making entry -_-

What...are you talking about