When Manny is about to enter the greenhouse again after he sproutella-ed the pipelines and Hector dies. The doors open a little. Then you see the shot of Manny, from a low camera, throwing down his gun and turning around/walking away.
This looks extremely simular to Marshal James Anderson's same move after he kills the Sawmill's endboss in Outlaws.
I'll get some screenshots later today. First gonna take a shower.