Why should we trust a smart contract? Well because smart contracts are stored on a blockchain, they inherit some interesting properties.
They are immutable and they are distributed.
Being immutable means that once a smart contract is created, it can never be changed again, no one is able to go behind your back and tamper with the code of your contract.
Being distributed means that the output of your contract is validated by everyone on the network, So a single person cannot force the contract to release the funds because other people on the network will spot this attempt and mark it as invalid. Tampering with smart contracts is almost impossible.