In The Tell-Tale Heart: Why is it ironic that the old man feared robbers?

In the Tell-Tale Heart, the situational irony occurs when
the old man (who is very rich), lives in torment fearing that robbers would go and steal
his gold and many possessions, so he keeps his windows and everything down to protect
himself.

Yet, the narrator has been growing obsessed with
the old man's eye which was cloudy and the narrator thought it was evil. The irony comes
when this same narrator kills the man because of this obsession. It is ironic because
many critics that have analyzed the story think that the narrator was either living with
the old man, or was familiar to him by the way that the old man reacted when the
narrator attacked him.

Hence, it is ironic that, as he
tried to protect himself from the outside world, evil was growing and getting ready to
attack him from the inside.