Sin against brethren not good.

1Co 8:12  But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ. 

1 Corinthians 8:12

But when ye sin so against the brethren - This is designed further to show the evil of causing others to sin; and hence, the evil which might arise from partaking of the meat offered to idols. The word sin here is to be taken in the sense of “injuring, offending, leading into sin.” You violate the law which requires you to love your brethren, and to seek their welfare, and thus you sin against them. Sin is properly against God; but there may be a course of injury pursued against people, or doing them injustice or wrong, and this is sin against them. Christians are bound to do right toward all.

And wound their weak conscience - The word “wound” here (τύπτοντες tuptontes, “smiting, beating”) is taken in the sense of injure. Their consciences are ill-informed. They have not the knowledge which you have. And by your conduct they are led further into error, and believe that the idol is something, and is to be honored. They are thus led into sin, and their conscience is more and more perverted, and oppressed more and more with a sense of guilt.

Ye sin against Christ - Because:

(1) Christ has commanded you to love them, and seek their good, and not to lead them into sin, and,

(2) Because they are so intimately united to Christ (see the notes at Jhn_15:1 ff) that to offend them is to offend him; to injure the members is to injure the head; to destroy their souls is to pain his heart and to injure his cause; see the note at Mat 10:40; compare Luk 10:16.  Barnes commentary