We study how the diffusion of being pivotal affects immoral outcomes. In a first set of experiments, subjects decide about agreeing to kill mice and receiving money versus objecting to kill mice and foregoing the monetary amount. In a baseline condition, subjects decide individually about the life of one mouse. In the main treatment, subjects are organized into groups of eight and decide simultaneously. Eight mice are killed if at least one subject supports the killing. The fraction of subjects agreeing to kill is significantly higher in the main condition compared to the baseline condition. In the second set of experiments, we run the same baseline and main conditions but use a charity context and additionally study sequential decisions. We replicate our main finding from the mouse paradigm and additionally show that in the sequential treatment, prosocial behavior is even less pronounced. We further show that the observed effects increase with experience, i.e., when we repeat the experiment for a second time. Finally, we report evidence on beliefs, elicited in our main experiments but also from a treatment of noninvolved observers, and show that beliefs about being pivotal are a main driver of our results.