Slayers are fighters by flavor, but they are not defenders. They can't mark or equivalent and they have very little control (they can start knocking things over at 7th-level, but that's about it).
I'm not saying they suck. They dish out a lot of DPS. They're just not defenders.
When did I say that they were? Did you actually read the portion of my post that you quoted? I asked if the character in question needed to be a defender. The OP mentioned shifting from DPS to tank. Slayer is a good way to do that while still maintaining much of the original class. Same armour, weapon, skill, feat choices.