You didn’t just say be mindful, you said that you could kill yourself by going vegan. That’s an extreme claim that your interlocutor met with an above average rebuttal.
B12 is important for vegans, but you will not kill yourself. Any of the deficiencies other than B12 that show up in vegans also show up in omnivores and carnivores, so it’s just as possible for some people, depending on their circumstances, to kill themselves out of a poor diet.