Instead of focusing on the results of war, let's look at our own motivations.

Up until this point, Arjuna has been talking about the destruction that war brings to families and society. However, in verse 45, his focus shifts from the outside world to something within himself. He laments, "Oh, what a terrible thing," and acknowledges that they are about to commit a great sin.

The Pandavas, who are originally known as beings with virtue and intelligence, may be trying to kill their relatives out of a desire for the throne and pleasure. Arjuna begins to see the very reason for fighting as "desire." At that moment, he realizes that the war is not an act for justice, but rather a great sin in which they are involved.

Japanese Translation from the Geeta Press Edition - Chapter 1, Section 45: "The Great Sin of Desire for Power and Pleasure"

We are foolish, we have committed a great sin. Those who strive to kill their own people out of greed for the pleasures of power. || 45 ||

Vocabulary

"Aho" is an interjection that expresses surprise.

"Bata" expresses deep sadness.

"mahat pāpam" means "a great sin."

"kartum vyavasita vayam" means "We decided to do it."

"rāja-sukha-lobhena" means "by desire for royal pleasures."

"hantum svajanāmudyatāḥ" means "the intention to kill one's own relatives."

Translation

"Ah, what a terrible thing. We have decided to commit a great sin. For the sake of desire for the throne and pleasure, we are trying to kill our own relatives."

Annotative Translation

The invariable word "aho" expresses surprise, and "bata" represents deep sadness.

By using these two things, Arjuna is trying to show the following:

The Pandavas were considered virtuous and intelligent people throughout the world.

That they were involved in acts of wrongdoing, was never appropriate.

Nevertheless, Even they decided to commit this terrible crime.

That was an extremely regrettable matter.

Arjuna mentions that, as a motive for this great sin, the desire for power and pleasure.

By doing so, he is showing that joining a war for such reasons would be a great mistake for them.