How can a Muslim say such a thing? You are being tested by Allah as this is why Allah created the creation and you are about to fail this test.
Are you better than the messengers of Allah?
Prophet Ayoub, peace be upon him was tested with the illness for eighteen years which made the people abandon him and throw him near the junk yard.
Take a good look at all the blessings Allah has given you and how you would
feel if you were to be deprived of them! You neglect all of His numerous favors and blessings yet you say “develop bad thoughts about Him”, instead!
Repent to Allah, show remorse and express your content to Allah while greatly asking Him for forgiveness and be patient and tolerant so that Allah may forgive what you had said!