run ansible command as another user using their ssh key
become_user will tell which user to become to run a particular ansible module on the remote server.

If you want to run the ansible command on your control machine as an other user, you need to use sudo or su on this machine e.g.

sudo -s -u ansible ansible all -m ping

The -s option will load the user's shell an make sure homedir and all other envs are correctly set prior to running the command.

An other possible solution would be to use the ssh key you created for the ansible user from your account

ansible all --private-key=/home/ansible/.ssh/id_rsa -m ping

This will potentially work only if your current user has read access to the given key.