I item build her mostly the same way, but I usually get Rylais first and not Rabadon's, as she's very squishy (as an assassin should be) So the extra survivability early really helps her. I tend to stay away from Mejais because of how squishy she is and because once i get a decent positive k/d I tend to get focused a LOT. I usually stick to tome, boots (i-ownian if doing well, sorc if not), rylais, rabadon's, then build according to enemy team; hourglass vs. ad carries, abysscepter if vs. ap, (usually abysscepter), or void staff/will of the ancients if doing moderately well and not being challenged.
You want to get as much AP on her as you can as early as possible in order to be effective and stay effective.. gunblade and other hybrid items don't really work all that well for her as they don't add enough AP or damage and since the majority of your damage is actually coming out of the combo of shunpo and lotus, which is mostly AP based. guinsoo's is a somewhat nice early game item, (for additional surprisebuttsecks ap damage) but a lot of it's effectiveness is lost later on when you need pure ap items and mag pen/cdr (which she needs BADLY)
One other thing that wasn't mentioned that takes some time to learn is that during team fights, you have to be very precise and patient with when you jump in with shunpo and death lotus. You want the enemy to have wasted their cc (hopefully all of it) and have at least one enemy low on hp before you come into the fight. This is because 1. She's squishy as hell, and 2. Her ult is channeled, and can easily be interrupted. You do NOT want to be the one to initiate, let your tanky dps do that. come in after most everyone has blown their spells and are waiting on CDs, then jump in and unleash. The best situation is one where there are several enemy heroes with low hp, then you can jump in, lotus, jump out, and have a trip, quad, or penta. She is a very opportunistic champ, and does in fact kill steal a LOT, so you'll just have to tell that to allied champs who didn't get enough gold early game. Their loss, not yours. You're adding to the team murder roster. That's your job. You have no CC so you can't really setup kills for your allies.
All this aside, you still need a decent team in order to win. I went 37/9 one game, but still lost because we rarely ever pushed turrets.
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in short, kat is a selfish hero that takes a lot from the team but doesn't add as much. it works in low level games, but having a hero like that in higher level games = GG, surrender at 20.