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Sammy killed that guy once, the one right after you have to defend that house (with Luis) from Las Plagas infected and then you get to chose a path between fighting El Gigante or a big group of infected (also before you ride the zipline thingie that let's you continue to the castle)
Sammy only did it to see if he was killable and cause he creeped Sammy out
I hated the part where you're stuck in the house as wave after wave of infected people try to get in. It took me six tries, and I learned to make good use of the Flash Grenades. Automatic weapons ftw!