From personal experience its electrocute
I used to run Deathfire touch because you couldnt even proc thunderlord in lane on old katarina (I was in small % of kat players who simply knew DFT is better dmg and consistency wise)
However electrocute can actually be proced and katarina can realisticly proc it once a minute
The longer CD with higher dmg fits her more
Also that gunblade buff + 3 damage KAPPA
the other reason is that katarina spikes harder with her first item harder then 95% of champions and games last 14 minutes
She is rather weak in lane. But that doesnt matter as long as she keeps up (10 cs less for example) she ll outspike with 1 item and given how much snowball there is that is enough to take over a game
People have to keep in mind that she used to run AD marks AP glyphs
Not everyone had luxury for her hybrid runes so that kept katarina underated