I see no textual proof that Lex's motives are as altruistic or noble as you're presenting them here; they could very well be entirely self-serving. An army to do his bidding for only his ambitions, the world be damned. Lex has not shown viewers anything to the contrary.
You seem to be forgetting the number of times Lex has used his money/influence, and even risked his own life, to help others...though I'll grant you that no one else in the SV universe seems to recall them, either. To mention just a few: Lex helped to rebuild Smallville after the second meteor shower (and possibly after the tornado at the end of the first season, though I'm not as certain of that one), and did the same for Metropolis after Zod's attack, for which he himself was in no way to blame (no matter Chloe's snide crack about 'guilt money'). He helped to save Smallville's largest source of jobs by saving the Luthorcorp plant, putting his own fortune at risk in the process. He walked into a situation where he was overwhelmingly likely to be killed in order to save a group of hostages, most of whom he didn't even know. Lex is capable of great nobility, and it is perfectly in character for him to sacrifice whatever he has to in order to save the world.
You assume, without a doubt, that...Lex should be this leader.
Lex is, quite simply, the only one who has stepped up on this -- the only one who has seen the looming threat and done his best to respond. Is the world supposed to wait until someone else -- someone with a higher approval rating, perhaps -- gets around to thinking about saving it? Lex is the leader here by default: there is no one else. Clark, Chloe and the proto-Justice League are actually part of the problem, given that they're all out undermining Lex's efforts.
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You seem to be forgetting the number of times Lex has used his money/influence, and even risked his own life, to help others...though I'll grant you that no one else in the SV universe seems to recall them, either. To mention just a few: Lex helped to rebuild Smallville after the second meteor shower (and possibly after the tornado at the end of the first season, though I'm not as certain of that one), and did the same for Metropolis after Zod's attack, for which he himself was in no way to blame (no matter Chloe's snide crack about 'guilt money'). He helped to save Smallville's largest source of jobs by saving the Luthorcorp plant, putting his own fortune at risk in the process. He walked into a situation where he was overwhelmingly likely to be killed in order to save a group of hostages, most of whom he didn't even know. Lex is capable of great nobility, and it is perfectly in character for him to sacrifice whatever he has to in order to save the world.
You assume, without a doubt, that...Lex should be this leader.
Lex is, quite simply, the only one who has stepped up on this -- the only one who has seen the looming threat and done his best to respond. Is the world supposed to wait until someone else -- someone with a higher approval rating, perhaps -- gets around to thinking about saving it? Lex is the leader here by default: there is no one else. Clark, Chloe and the proto-Justice League are actually part of the problem, given that they're all out undermining Lex's efforts.