When you are sinning and your spirit knows it, your flesh will try to find excuses to make the sin seem legitimate and okay. There are many ways the flesh can do this, but one of the trickiest ways is trying to excuse your sin by pointing out the perceived sins of others. There are 2 main ways someone can do this:
1) You can point out the actual sins of others. "X does Y sin, so I can do Z sin."
2)You can point out the Old Testament laws people don't follow, and say because those laws aren't followed then who is to say what you're doing is still a sin? "Well, people eat pork and wear clothes with mixed materials, so it's okay for me to do Z sin."
Both methods of argument are flawed and faulty thinking used by the flesh to keep you sinning.
Dealing with Point 1, your main concern should not be how others are sinning. Your main concern is how you are sinning, and how to fix it. Sins are not like negatives in math to cancel each other out. Just as Jesus says you cannot judge another's sin without first fixing yourself, the inverse works as well: you cannot excuse your sin by pointing out the sins of others. God will judge you based on your life alone. Holy Spirit will only be able to work well in a clean temple, and your temple does not become clean by pointing out the sins in others.
Now on to Point 2. First of all, let me make a few clarifications. Each person is required to live according to the truths revealed to them from the Bible. When a new truth is revealed, the person is then required to living according to that truth. For the point of this discussion there are 2 types of truths revealed to Christians. There is the truth (or belief) revealed to one group of Christians that the OT laws are still in affect today, and then there is the truth revealed to another group of Christians that specific OT laws are no longer in affect today. All else being equal, both groups have these beliefs based on the truths revealed to them. We will not go into whether these groups are right or wrong, because that is beside the point. The point is that both groups have scriptural foundation for their beliefs.
Now that we've clarified that point, I can move on to actually dissecting Point 2. For the Torah-following Christians, you cannot use the argument of "well, they don't follow those laws, so I can do Z sin" because they do follow the laws. Simple. For the nonTorah-following Christians, they do not follow Torah based on a very specific understanding of the scripture, and it is the ceremonial laws only they no longer follow. They still follow all the moral laws. As such, their understanding of the truth dictates they are not sinning by not following specific laws. This means that unless the sin you are doing is ceremonial (and as such isn't actually a sin) then you can't compare it to the perceived sins of others.