Actually, with the right build Kayle is probably the strongest AD carry late game. Before her rework, she had an incredibly shitty early game to make up for being the strongest end game character due to double scaling from her old passive. With her new passive, you get an additional 15% shred to both defense. With her Q scaling at 1 AD + constant up time on her E, you're just better off going full AD build on her.

She scale incredibly well with lifesteal due to her ult being a 3 second invincibility. So you can get low, pop ult and lifesteal your way back to full in 3-4 hits.

I suggest starting with 2-3 Doran Blade and then transition into an early Bloodthrister + Zerker. What you build next is situational, but Black Cleaver > Malady > Phantom Dancer/Nashor's Tooth/2nd BT and then finish off with a Guardian Angel is my usual standard path.

Start with one point in E and then max R > Q > W > E. A fun first blood trick with her is to run Surge/Ghost and lane with someone like Nunu or Taric as your support. Her damage output is ridiculous with Surge because her abilities also scale off AP so you're getting a bunch of free damage. A well placed stun from taric or a slow + AS boost from Nunu and you're pretty much guaranteed to melt their AD carry or Support.