Why Thanos is after Infinity Stones when he without them is still most powerful Marvel being out there and can easily kick asses of all avengers without much trouble. In my opinion answer of this question is that Thanos is not gathering the infinity Stones for the purpose of defeating the avengers. He doesn't need them to do so. He is much more powerful than any one avenger and probably more powerful than all of them combined.
Thanos is gathering the stones so that he can, in his own words, “balance the universe.” The infinity stones, when used in conjunction through the infinity gauntlet, provide one with the abilities of God. The wielder can alter reality in whatever form they please, teleport, and will things in and out of existence. In the comics, Thanos’ goal is to kill half the known life in the universe to impress his lover, Death.
There is another theory that since to cure his God Cancer. As not a lot of people know this that Thanos is gradually dying of due to God Cancer. Therefor collecting all the Infinity Stones will cure him of the God Cancer. So he is not collecting Infinity stones out of love for his lover death, who is already in love with Deadpool so making Death fall in love with him would not have been an easy task anyway.
Whatever the true motive Thanos have behind collecting Infinity Stones but one thing is pretty clear that all he want is destruction and there is not confusion about that.