Jaryuu Tensei

A loser salaryman who always says "I wish I could win the lottery..." is reincarnated into a world where he is the strongest evil dragon. He is very content to live his life in a forest with slimes that look up to him. One day, Evil king's subordinates suddenly come attacking him and cruelly kill the slimes. He gets enraged, and avenges the deaths quickly, however, decides to leave the forest in order to never have any more casualties. Weak but kind and sentimental, a former loser salaryman begins a carefree adventure fantasy!

(Source: Alpha Manga)