It's our reaction to our old kamma unskillful thoughts that can cause bad present kamma
Question: If any unskillful thought arises and you acknowledge it as unskillful, does it still have negative kammic effects? Thanissaro Bhikkhu: No. Question: In other words, does the arising of unskillful thoughts cause bad kamma or is it just our reaction to them? Thanissaro Bhikkhu: It’s our reaction to them that can cause bad kamma. The fact that the thought arises is the result of old kamma. What you do with it is your new kamma. If you simply acknowledge it and it goes away, or if you think skillful thoughts that counteract it and make it go away, then the new kamma is good new kamma. ~ Good Heart, Good Mind: The Practice of the Ten Perfections