The answer: It's not. At least, the kind that makes up 78% of the atmosphere is not bad. The problem arises when this nitrogen takes other forms, including NO2, NO, and nitrates. We call these reactive nitrogen, and excess amounts of these have a number of adverse effects, including eutrophication of water sources and magnification of the greenhouse effect.
Nitrates in bodies of water result in excess nutrients in these bodies of water. Excess growth results in plants such as algae blooms which are decomposed by bacteria who consume the oxygen of the body of water. Thus, eutrophication often results in anoxic conditions, which kills fish and other organisms and effectively desecrates marine environments.
Furthermore, the the combustion of fossil fuels releases nitrous oxide (N2O) into the atmosphere, which is a greenhouse gas and therefore magnifies the greenhouse gas effect that is so crucial to the climate crisis. If we can reduce the amount of nitrous oxide that is emitted into the atmosphere, we come one step closer to reducing the climate crisis.
Therefore, nitrogen by itself is not "bad". It's what happens when nitrogen combines with other elements to form reactive nitrogen (ammonia, nitrous oxides, nitrates, etc) in excess quantities that has negative impacts on the environment. But atmospheric nitrogen in its elemental form (N2) poses absolutely no threat.