He faked being Liquid because the AI recognized Liquid as an old threat they had previously neutralized through a simple method: send in Solid Snake and let him do the job. Had they known they were facing Ocelot in his "right mind" they would've taken an entirely different and much more threatening approach since they knew that Ocelot was capable of more. The thing is, before MGS4, Ocelot was actually the Patriots' main head-director of their PMC armies. By acting as if Liquid finally took over him, he can act against the AIs without taking the fall for it identity-wise (yes it's the same target, but making them think it was Liquid actually put their guard down, so much so that they thought sending Solid Snake was good enough to solve the problem).
Honestly I wish they actually mentioned that part of the story more clearly, because it would've shown exactly WHY taking on the Liquid persona made sense in the way that it would've made the reveal seem less of a heroic act and more of a tactical one. Actually that's the whole point of Ocelot taking over GW with the whole "Guns of the Patriots" gimmick, he purposely hijacked the military system of the AI to remove any possible way for the AI to send in anything against him (except Snake which had the virus anyway). He was literally cutting as much ties from the Patriots all while acting as if he was an old threat they already taken down before.
Actually this is where we differ in opinion towards Ocelot. Ocelot may have had good intentions in freeing Big Boss, but I never saw him as a hero especially when his main plan was to unleash anarchy upon the world (something I'm sure even Big Boss wouldn't have wanted). You ought to realize that Ocelot always had an anarchical goal from the start, it's why he actually didn't mind supporting Liquid in MGS1 (he even has a speech about this in MGS1), except Liquid failed so he bailed out and made it seem like he was still with the Patriots. What he was plotting in MGS4 was a seemingly full-proof plan that would ensure that the Patriots would get destroyed no matter what (be it killing the AI with a nuke or having Snake upload the virus). Either way, he was expecting to achieve anarchy, but what he didn't know was that Sunny had actually modified to virus in order to keep society's resources intact, thus preventing the "anarchy" side of Ocelot's plan from happening. He didn't know about this, so he died thinking he achieved what he wanted though, hence why he was acting all triumphantly. In the end it's the "next generation" the saved the world I guess and not really Ocelot (he just had a HUGE helping hand in it even if he plotted something much worse).
Actually, the idea of pushing aside the whole arm-thing and Ocelot actually fooling the Patriots was decided on by Kojima, was most likely because he was criticized over the Liquid-arm thing in the first place (I recall he got a lot of flak for it). Not to mention, I wouldn't be surprised if Ocelot proved to be a more favorite character for Kojima, especially since he once said that the creation of Liquid as a "dark clone" felt like a cheap tactic at making an enemy "equal" to Snake (I don't share this opinion though, I thought Liquid was great in MGS1). Still, as convoluted as MGS4 was (though the plot wasn't its only problem), I'm happy with the end result of Snake facing off Ocelot for the final battle (again, son of Big Boss vs sone of The Boss) instead of just a retread of MGS1, which would've felt like an overblown nostalgia-fanwank (this is just my opinion). I mean, all times the player faced Ocelot you never actually beat him properly. Yes you beat him gameplay-wise, but the ending cutscene always shows the fights being drawn to a stalemate, so actually defeating him in the end actually created that perfect sense of closure for the character in my eyes.
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Totally agree with ronri
in the end you get used to it, but it still sucks.
