Buybacks (like dividends) shrink a company, say ZZZ, by distributing excess assets to shareholders. This does have the effect of making it more affordable for you to acquire a given stake in ZZZ, at least if buying outright. But if your purchase is financed, the amount of leverage the lender allows will be affected by due diligence on ZZZ's balance sheet, and as ZZZ's leverage increases after a buyback, your allowed acquisition leverage may decrease, making it a wash. At the levels being discussed, this would be a custom deal with an investment bank, not a simple margin account with a broker.
When you own all outstanding shares of ZZZ, you not only have control of ZZZ (in fact, you get this with 51% of outstanding shares); you own all of ZZZ (including the treasury shares). Thus, to "damage" ZZZ and destroy the value of your investment would be cutting off your nose to spite your face. Instead, you would probably want to extract value from ZZZ's assets by integrating its business and customer base with yours, gaining economies of scale, etc.
In some cases, such an acquisition could be deemed predatory or anti-competitive. Maybe you really do want to shut down ZZZ's business if it threatens to make your main business obsolete in the future (this can occur in the tech industry). If you have a dominant market position, such an acquisition may be subject to government antitrust review.
So, yes, you might have an easier time taking over ZZZ for nefarious purposes because ZZZ did a buyback, but you will still be subject to regulations, and your ability to borrow money to buy ZZZ may go down.