[ue_]

Why would that make it more secure? Couldn't I simply make a lot of fraudelent transactions and append them, having a long chain, but a false one?

[gtfierro]

You could, but you'd be competing with everyone else that's also making transactions so you'd need an incredible amount of computational power to outstrip that.

[nickporter]

And if you had that much hashing power, you'd be better off using it for mining. It's explained in more detail in the (surprisingly accessible) bitcoin whitepaper.