What is a single word to use in place of 'Costing a large number of lives'?

Deadly would fit quite well.

AHD

1. Causing or tending to cause death

It is used in similar situations:

The Rwandan genocide likewise ended with a military solution, in this case a Tutsi victory. (But this became the seed for the most deadly war on earth since 1945, the Congo War of 1998 -- 2006.) (Game Theory (Stanford Encyclopedia of Philosophy))

Vietnam veterans Antonio Porres and Frankie Infante also enlisted together, fought the length of that deadly war and miraculously returned intact. ( Houston Chronicle , LORI RODRIGUEZ, HOMES VS. HERITAGE;; The relationship between a subdivision and a cemetery is side-by-side but hardly eye-to-eye)

OUR COUNTRY HAS BEEN AT WAR on two fronts, but only the war abroad was declared. It is time for you and me to make our leaders confront the equally deadly war raging in our cities, towns, and rural and suburban areas, which poses as much of a threat to the nation's security and economic prosperity as any foreign enemy. # Every time a cruise missile landed and each time an AWACS plane took off in the Persian Gulf, it was hard to restrain myself from shouting: " My God, how many Head Start slots was that? How much money in housing for homeless children could have been provided?" (Mother Jones, Edelman, M.W., Kids first! (cover story), Vol. 16 Issue 3, p31)

"Fatal," "lethal," "destructive," "death-dealing," "mortal," "murderous," "homicidal," "bloody," and many others would work in this situation.

These are the applicable definitions of the words I listed above:

All of the following definitions are from [Lexico]

Fatal - "Causing death."

Lethal - "Harmful or destructive."

Destructive - "Causing great and irreparable harm or damage."

Death-dealing - "Capable of causing death."

Mortal - "Causing or liable to cause death; fatal." OR "(of a battle) fought to the death."

Murderous - "(of an action, event, or plan) involving murder or extreme violence."

Homicidal - "Capable of or tending toward murder; murderous."

Bloody - "Involving or characterized by bloodshed or cruelty."

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There are many other words that could be applied to this situation. Run any of these words through a thesaurus and you will find many alternatives. Also, the use of the word "wasteful" is disrespectful in this context; you should remove/replace it.