Romans 2:14-15 — … For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another).
The Gentiles, without the law, still have consciences that make them feel guilt, but they make excuses to justify themselves.
Nature = natural impulse (in contrast to obedience to God's decrees).
The law isn't written in the Gentiles' hearts, but the work of the law is, namely condemnation.
Conscience = knowledge of right and wrong. Conscience is to Gentiles what Law is to Jews, a guide for individual conduct and for relations with others.
Thoughts = reasoning
Accusing and excusing both indicate guilt.
There is no justification in this passage, only condemnation.