According to the Mahabharata, this battle took place between his cousins and rulers of different kingdoms, the Pandavas and the Kauravas. The Pandavas won the battle, but many warriors on both sides lost their lives.
Regarding the specific characters you mentioned, Dronacharya, Karna and Duryodhana, their death in battle was not necessarily "cowardly" or against the rules of Kshatriya Dharma. Each of these characters had their own personal histories and conflicts that led to their involvement in combat. Dronacharya, for example, was a respected teacher and mentor to many warriors on both sides. However, he was also deeply loyal to the Kauravas and fought alongside them in battle. His death was the result of the Pandavas' strategy to mislead him about his son's fate on the battlefield.
Another warrior who fought for Kaurava, Karna had a complicated upbringing, including being abandoned at birth and later discovering his royal heritage. He was known for his prowess in archery and was a formidable opponent to the Pandavas. His death in battle was the result of a duel with Arjuna, which Arjuna ultimately won.
Duryodhana, the main opponent of the Mahabharata, was known for his arrogance and pride, which often resulted in him making bad decisions.His death was the death of the Pandavas, who had a long-standing rivalry with him. It was the result of a confrontation with one of his, Bhima.
The events of the Battle of Kurukshetra are viewed by some as tragic or unjust, but overall the complexity of the characters and their motivations makes it difficult to make conclusive judgments about the actions of any particular individual during the battle. It's difficult. Ultimately it depends on your personal interpretation and beliefs.