Contrary to popular belief, angels have no incentive to steal an idea they've been pitched on. More accurately, doing so is a net negative.
Angels do a few things well: quickly vet a flood (sometimes dozens a day) of inbound deals to back with a tiny investment, and support the companies that have found product market fit after the first investment. Both of these sound trivial, but it's difficult to do in practice and both are incredibly valuable. The vast majority of angels are either not good at building things, or they're too busy/exhausted to do so with their time. If they were, they'd become entrepreneurs, or join as an exec in the most successful startup they backed. Also, the great networks that angel investors provide are on the business side. Angels expect you, the technical founder, to have the technical network and skills to hire and build product.
So, angels exist to source good deals, and support the winners as they surface. This is their lifeblood. To steal an idea is a cardinal sin in the angel community as much as the entrepreneur community. They'd be outcast, removed from deal flow, and their career as a seed investor would cease to exist. If we assume that the majority of business ideas are flawed, and the majority of startups die, then for an angel investor to steal an idea makes no sense: they'd be effectively ending their careers for little upside.
On a more humorous note: being an angel is an easy life compared to building a business, let alone building a business off of someone else's idea. Any angel that would give up an easy life that makes money for a hard life of executing an idea they aren't passionate about, and a life with a high risk of failure, is an idiot.
