If gabriel can be killed with a エンジェル blade why cant lucifer i know this is a old 質問

Lucifer probably could have been killed using an エンジェル または archangel blade, but no one thought of using one against him.

it could be that he was an エンジェル but he have become a demon so he might be a dangel now and an エンジェル blade won't kill him

He's pretty damn fast and strong. Gabriel was an archangel and still couldn't beat him, maybe Micheal could though. Having the blade doesn't mean あなた can get a hit in. It wouldn't be as easy as with Zachariah.

Normal エンジェル blades don't work on Archangels so they have special blades (Archangel Blades) that can kill other 天使 but are the only thing that can kill each other. Lucifer cannot be killed with one of the normal エンジェル blades since he is an Archangel and he used his Archangel blade to kill Gabriel. (don't worry I thought about this a lot when I first watched Supernatural)

Gabriel wasn't killed with an エンジェル Blade he was killed with his own Archangel Blade which is もっと見る powerful and rare to come によって since only Archangels possess them.

I think it's because certain fallen 天使 can't be killed , like Lucifer.