If God is so powerful why did he let Satan go?

One answer

God gives all the freedom of choice - including Satan.

Another answer

If there is no Bad Guy, then what's the value of a Good Guy?

Another answer

In the first instance when Satan rebelled, we may never know the answer, as this has not been revealed. However it is only true for a time as His doom is sealed as Hell has been prepared for the Devil and his angels, where he will remain, once finally sent there.

Answer:

Satan was not let go, he and 1/3 of the angels who rebelled against God were cast out of Heaven.

His final doom in the lake of fire is promised by God and God cannot lie.