” קץ כל בשר בא לפני כי מלאה הארץ חמס מפניהם”
“The end of all humanity has come before me, for the earth is filled with robbery through them”
(6:13)

Rashi explains, the generations fate was sealed only because of the sin of robbery why is this?
HaRav Yosef Shaul Nathanson (author of Shoel U’Meishiv), explains this with a story, there once was a pelican searching for food over the water he dove into the water and caught a big fish in his beak, he was about to swallow his meal. At the last second the fish started cry please have mercy let me go. The pelican was about to drop the fish back into the water, just than a smaller fish fell out of the bigger ones mouth. The pelican said you’re a fake you foolish fish you expect me to have mercy on you when you cant have mercy on others.
No matter how big the generation sinned HaShem was ready to show mercy. The fact that they themselves couldn’t give mercy to others and stole from them, made them lose the privilege of mercy from HaShem, for that reason the sin of robbery sealed there fate.