ano jigoku kara no kuroi neko wa ningen no chi o nomu koto ga dekiru yo
That hellspawn black cat over there can drink human blood!!
(Even if the sentence is correct would it be said in a very surprised tone?)
This sentence is OK, grammatically correct. Well done!
If anything, I would change the ending of the senteince to
dekiru 'nda (yo)!
if it is to be said with a tone of surprise.
"nda" is contracted form of "noda", it's to emphasize the sentence, adding feelings.
ano o mireba, shinu yo
If you look at that you'll die
This sentence is also good, except "ano".
Unlike that in English which can stand on its own as well as to modify noun, "ano" is specifically to modify noun. It should be "are".
kono hana (this flower) = kore (this)
sono eiga (that film) = sore (that)
ano hon (that book) = are (that)
dono ie (which house) = dore (which)
ningentachi o koroseba, sugoi na ja nai yo
If you kill humans that won't be amazing.
The first part is OK, the latter is awkward.
Sugoi is i-adjective, so its negative form is "-ku nai", not "ja-nai/dewa nai".
Sorry I don't fully understand the connotation of this sentence, but I might say "sugoku nai (yo)".
Depending on the meaning of "if..." part, the first part might need to be modified.