Does Venice have a law against shedding "human" blood, or specifically "Christian" blood?
This bond doth give thee here no jot of blood;
The words expressly are, a pound of flesh
Take then thy bond, take thou thy pound of flesh;
But, in the cutting it, if thou dost shed
One drop of Christian blood, thy lands and goods
Are, by the laws of Venice, confiscate
Unto the state of Venice.
Portia seems to suggest that the law is against shedding "Christian blood", though very often in this play "Christian" and "human" are considered synonyms anyway.
However, that Venice confiscates the lands and goods of anyone who sheds Christian blood might be seen to suggest that shedding Shylock's blood would not be illegal - you might even see the law itself as potentially promoting anti-semitism: of course, the second statue Portia cites is specifically anti-foreigner:
It is enacted in the laws of Venice
If it be proved against an alien
That by direct or indirect attempts
He seek the life of any citizen...
The text of the play, though, seems clear that it is "Christian" blood that you are not allowed to shed.