The answer is obvious, it is because we are not exactly the God, we are jiva, an energy of God. And because we are an energy of God, we are called as God by some. Just like fire and a spark of fire are essentially the same, so also God(Brahman) and us(Jiva) are essentially the same. But, just like fire is huge than a spark of fire and has more heat or energy, so also there are differences between us and God. Hence, to say we are God would not be absolutely correct. Now let's talk about the cause of corruption, violence, etc.
Apart from jiva shakti (which is us), there is another kind of energy of God known as maya shakti (illusory potency). This maya is also known as prakruti whose product are the three modes of material nature known as satva(goodness), rajas(passion), tamas(ignorance). And the major difference between God and us is that, God is mayadhish (the Lord of maya)[Sve. Up - 4.10] but we are mayadhin (under the influence of maya). So maya cannot even stand before God ashamed of Her position, but those who are under it's influence get bewildered by it:
vilajjamānayā yasya sthātum īkṣā-pathe ’muyā
vimohitā vikatthante mamāham iti durdhiyaḥ [SB - 2.5.13]
The illusory energy of the Lord cannot take precedence over the Lord, being ashamed of her position, but those who are bewildered by her always talk nonsense, being absorbed in thoughts of “It is I” and “It is mine.”
So being influenced by the three modes of material nature in the course of time we get affected by corruption, viloence, etc. If we were God, then we would be free from these effects. But because we are not we are afflicted by these things and only by approaching God that we can get moksha, liberation, etc. which is getting rid of this divine energy of God known as maya:
daivī hy eṣā guṇa-mayī mama māyā duratyayā
mām eva ye prapadyante māyām etāṁ taranti te [BG - 7.14]
This divine energy of Mine, consisting of the three modes of material nature, is difﬁcult to overcome. But those who have surrendered unto Me can easily cross beyond it.
Now there are certain schools of thought (advaita of Shankara for example) that say jiva brahmaiva na parah. That is, jiva is Brahman itself, not different. And followers of that tenet strongly cite statements like aham brahmasmi,etc. to say we are Brahman or God. But one should clearly understand what kind of Brahman we are and who are eligible to say I am Brahman. And secondly, saying that jiva is Brahman itself is only a partial view of the truth. In the Vedas there are both kinds of statements which say jiva is Brahman and jiva is not Brahman. Hence, different schools of thought have arisen in course of time giving importance to different statements of the Vedas. But the truth is, there are both similarities and disimilarites between God and us (I am skipping verses to keep the post short) and we are God only in the sense that we are energies of God [BG - 15.7]. But because we are under the influence of different modes of nature caused by maya, we act in corrupted ways and need liberation.