Would this have been legal? I have this vague idea that beating a slave to death would have been criminal in some places, if you had white witnesses who could testify that it happened. So the ambassador may actually be a witness.
I doubt Preston Brooks would do it. It wouldn't be in character for him, or for a slave. His crazy violent attack on Sumner is the kind of thing people do when they feel powerless, especially when they aren't used to it--in his case, he felt powerless to defend his family against what he perceived as a grave insult, because Sumner would have refused to duel. If a slave insulted his family, he wouldn't feel it as much as an insult, because as a Southern racist of the plantation class he wouldn't take what a slave said seriously, and it wouldn't be nearly as public as Sumner's speech; plus he wouldn't feel powerless, he'd just order the slave whipped. And a slave is very unlikely to insult him either, because they weren't stupid.
So I think it needs to be a drunkard, and maybe someone who had just experienced a severe blow and who was already a mess emotionally anyway. Certainly possible, though its harder to see why the Ambassador would associate with such a person.