Does Suzanne Collins justify the use of violence in the novel "The Hunger Games"?

Collins presents the use of violence in several different ways in her novel, but most importantly, she uses other elements to show how differently violence can be perceived when it is combined with different motives.
For example, Collins presents what violence looks like when it stems from a misuse of power. The Hunger Games themselves are an example of this: an annual televised fight to the death among children, provided namely for gruesome entertainment among the country’s sovereign district The Capitol, but also to ensure that their position of power over Panem remains intact and undisputed.
Collins clearly never tries to justify the Capitol’s use of violence, but instead completely villainizes it so that it becomes a horrific and unjustifiable obstacle in her heroine’s way.
However, Collins also presents violence in a way that doesn’t seem so belligerent and horrific, and would understandably stir up questions like yours.
When Katniss is put into situations in which she feels forced to use violence as her only way out of a high-stakes situation, Collins presents it in a way that can help a reader sympathize with her heroine’s hard decision. For example, in Chapter 14, in the arena, Katniss is trapped at the top of a tree while the Careers – a group of her opponents – rest below, waiting until morning to kill her. In this situation, her thoughts reveal the following:

“Darkness has given me a brief reprieve, but by the time the sun rises, the Careers will have formulated a plan to kill me. There’s no way they could do otherwise after I’ve made them look so stupid. That [tracker jacker] nest may be the sole option I have left. If I can drop it down on them, I may be able to escape. But I’ll risk my life in the process.” (Collins, pg. 486)

Her hopelessness is especially apparent when she knows that injuring or killing the Careers with the tracker jackers could result in her own death as well. She truly believes in this moment that a violent attack on the Careers is her only option.
After she follows through with this plan, Katniss is delirious from tracker jacker stings and is thinking only about one thing:

“The swelling. The pain. The ooze. Watching Glimmer twitching to death on the ground. It’s a lot to handle before the sun has even cleared the horizon. I don’t want to think about what Glimmer must look like now. Her body disfigured. Her swollen fingers stiffening around the bow . . .” (pg. 488)

Katniss’s attitude toward violence is not to justify it in her own hands, or to relish a successful violent attack against her enemies. She does not dismiss her targets, even if they never had her interests at heart, and she never appears to enjoy or revel in hurting or killing others.
This is shown in almost symbolic ways throughout the novel, such as in Chapter 18, when Katniss places flowers around her friend Rue’s body in the arena after she was wounded with a spear and killed. By doing this, Katniss is trying to find a way to soften the blow that is seeing Rue with a bloody hole in her abdomen. She surrounds her with nature, trying to bring peace to the aftermath of a gory death. This event is supposed to touch readers emotionally. It’s supposed to make them feel the grief that Katniss feels and stoke anger toward her situation. It's supposed to remind them of the consequences of violence.
It is also shown in the few moments before Katniss’s mercy killing of Cato in Chapter 25, after he was attacked by the Capitol mutts. She’d thought beforehand:

“The cold would be torture enough, but the real nightmare is listening to Cato, moaning, begging, and finally just whimpering as the mutts work away at him. After a very short time, I don’t care who he is or what he’s done, all I want is for his suffering to end.” (pg. 790)

And that last line is one of the most important points to note in this answer, because it proves that Katniss does not enjoy the suffering of her most troubling enemy in the arena. The violence is never something she believes anyone deserves.
And in the end, when Katniss decides to rebel against the Capitol instead of killing Peeta, she doesn’t veer toward violent thoughts or plans of gruesome revenge toward others, but instead decides to threaten the Capitol with a strategic double suicide that would ultimately overthrow the power they’d held over Panem with the Games. It was, in her situation, probably the least violent thing she could think of.
Ultimately, it is shown through her main heroine that Collins’ goal was never to justify the use of violence in her novel but rather to condemn it. She manages to create such effective tension in her novel with violence only because she’s managed to create a world where violence is not meant to be satisfying, like in Chuck Palahniuk’s Fight Club. It’s not meant to be an anticipated expression of vengeance, like in Vince Gilligan’s Breaking Bad. Violence in The Hunger Games is never an effort to prove that violence itself can be justified, but instead it is trying to uphold a character’s moral integrity by defining a justifiable motive when violence becomes the only way out. The novel is, by all means, an indictment of violence and the damage that it can create in a world where it is the most present evil of all.