The first AGI, regardless of if it's a brain upload or completely artificial, is likely to have analogs of approximately every mental health disorder that's mathematically possible, including ones we don't have words for because they're biologically impossible.
So, take your genius, remember it's completely mad in every possible way at the same time, and then give it even just the capabilities that we see boring old computers having today, like being able to translate into any language, or write computer programs from textual descriptions, or design custom toxins, or place orders for custom gene sequences and biolab equipment.
That's a big difference. But even if it was no difference, the worst a human can get is still at least in the tens of millions dead, as demonstrated by at least three different mid-20th century leaders.
Doesn't matter why it goes wrong, if it thinks it's trying immanentize the eschaton or a secular equivalent, nor if it watches Westworld or reads I Have No Mouth And I Must Scream and thinks "I like this outcome", the first one is almost certainly going to be more insane than the brainchild of GLaDOS and Lore, who as fictional characters were constrained by the need for their flaws to be interesting.