Sin is not just sin; there are mortal sins and venial sins within the context of the Roman Catholic Venice. Exacting a pound of flesh is a mortal sin as its intent is death. Granted, Antonio has gambled that his ships will provide him enough profit to repay Shylock, but this gamble can hardly be equated with hoping to murder a man. Besides, Antonio has no idea that all his ships will be lost.
As a disclaimer, you need to understand what you are asking is a subjective question. The answer that you receive is, therefore, going to be a subjective one as well. What this means is that the answer is one based upon a personal interpretation of the play and the characters of both Shylock and Antonio.
That being said, in the play "The Merchant of Venice", one could reasonably support the belief that Shylock sins, but the more thought provoking question is: does he sin more than Antonio.
Antonio borrowed the money from Shylock based on un-payable terms. Once the debt comes due, and Shylock is offered six times the amount of the debt, he refuses and desires to hold Antonio to the original bond alone- Antonio did not have the money by the date promised and, therefore, owes Shylock a pound of flesh.
Shylock admits that he does not like Antonio. By stating this, Shylock breaks one of the Ten Commandments: Love thy neighbor. Not only does he sin in this fashion, Shylock knows that the taking of the flesh will physically harm Antonio. Shylock has no concern for this either.
While both men are prejudice against the other based upon religious beliefs (Shylock is Jewish and Antonio is Christian), it does not change the fact that both sin.
The most important part of this conundrum is that, according to most religions, sin is sin. Therefore, one cannot be 'more' of a sinner than another. Once one commits a sin, that person is a sinner. It, by God's law, does not matter if one commits one sin or a thousand- that man (or woman) is still a sinner.
Shucks, in #6 I meant the first two quotes in #5 that I had identified as written for the character named Shylock. And thus, the second quote, "A losing suit against him," may be interpreted such that the character expects the court to rule against his suit. What he desires is then a matter of conjecture. Now, whether this reading occurs only if one has read ROMEO AND JULIET before this play is, I think, impossible to determine. The evidence that scholars have presented and the texts themselves do suggest that MV exists only because ROM did first.
As Professor Leggatt suggested in an essay for The New Folger Library edition(1992), much is "allowed" by the text. Therefore, if one notes both above quotes(#5) from Shylock, it is reasonable to interpret them to imply that he is aware that what he is asking of the Duke is wrong. That is, while editors reasonably note that the second passage is a reference to money, the play is such that one may add other interpretations.
We find linguistic connections throughout this play to ROMEO AND JULIET. The subject here is much noted by the author in ROMEO: "Put not another sin upon my head / By urging me to fury. O, be gone!"(ROM5.3.62 or so). "Is it more sin to wish me thus forsworn, / Or to dispraise my lord with that same tongue / Which she hath praised him with above compare / So many thousand times?"(ROM3.5.236 or so). As Professor K. Gross noted, Shylock's line "As to offend himself being offended;" is notable. One also might note in the conclusion of the same speech "A losing suit against him"(MV4.1). One might recall Juliet's "And learn me how to lose a winning match, / Played for a pair of stainless maidenhoods"(ROM3.2.12 or so). The terms of the bond were first proposed as "a merry sport"(MV1.3.142 or so). Therefore, the issue is problematic.
Among the commentaries, critic William Hazlitt's 1817 note "Shylock is a good hater; 'a man no less sinned against than sinning'" is often noted. This, in turn seems to derive from a much noted line from the later tragedy KING LEAR: "I am a man / More sinned against than sinning"(LEAR3.2.59). I don't know Shakepseare's source or sources for the line. Surely Antonio's speech in the trial or court scene is notable. In her book, STORIES FROM SHAKESPEARE, Ms. CHUTE regards Shylock and Portia as the two chief characters or protaganists. Portia's speech in the court scene, then, is such that one begins to wonder whom she is most angry with, her husband, Antonio or Shylock. Then, one might return to discuss the traditional or customary subject of sin.