So, people deposit money with you, you agree to pay them fixed % back, and then you invest money for your own profit?
Sounds like a bank to me. Or a dividend-paying mutual fund, or a number of other similar ideas. Sure, you're welcome to start something like that up; it's unlikely you'd get money from people unless you had already proved yourself competent as an investor, though. After all, if it's possible to get a safe, comfortable 5%, why would anyone give you money at 2% or 3% instead of just getting that 5% for themselves? Or, more likely, finding a competitor who gives 4.9%?
As in most things, the market will find inefficiencies like this and squash them like bugs.
That said, there are some opportunities that take advantage of other kinds of situations. The most common I'd say is owning a home. You take out a mortgage for 80% of the value of the home, instead of buying it outright, and you instead invest the 80% in the market. You likely will beat your mortgage rate given the current 3.5%-4% rates. That's effectively doing the same thing, with an expense you'd have anyway (you always have to live somewhere); since it's a secured loan, you get better rates than the market will give (as it's quite safe) and you can use tax laws in your favor (mortgage interest deduction, in particular, and the capital gains break on primary residences).