Birkin was never loyal to Umbrella, and the only reason he stayed on Umbrella and didn't leave with Wesker was because he wanted to finish the G project. I don't remember exactly, but he was either planning on leaving once he was finished with G or planning on selling it, one of the 2. Umbrella caught wind of this, which is why they sent soldiers to N.E.S.T. I don't remember Umbrella soldiers trying to keep him safe, he got attacked, injected himself with the G virus to not die and exact revenge. I'm pretty sure no Umbrella soldiers tried to keep him safe, but it's been a while so I might not be remembering it right.
Well I don't know about any soldiers, but I remember at least in the original RE2, you can find a note in an Umbrella facility that says something along the lines of "When Birkin is in the building, there is a high probability he will be attacked. You need to stay on alert and keep him safe."
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