the revolutionaries were prepared to fight for their country, even though they thereby risked mass death

كان الثوريون مستعدين للقتال من أجل بلادهم على الرغم من أنهم بذلك يخاطرون بالموت الجماعي

يخاطرون بمقتلهم جميعًا

موت جماعى

English term (edited): though they thereby risked mass death

In English, the transitive verb "to risk" can be used in two different ways:

1. To do something that could lead to the loss of something, e.g., "she risked her life."

2. To do something that could lead to an unwanted outcome, e.g., "she risked her death."

As you can see from the above two examples, one cannot tell which meaning ins intended until one has read the object.

In the clause "though they thereby risked mass death," it is the second meaning that is intended.

In Arabic, the verb يخاطر بكذا (or يجازف بكذا) does not have this ambiguity about it. The prepositional object is always the thing one fears to lose. Therefore, in the clause "though they thereby risked mass death," you must replace "death" with its opposite:

رغم أنهم كانوا بذلك يخاطرون (أو يجازفون) بحياة الكثير

If you wish to keep the word "death," you need to play a little trick:
رغم كون ذلك مخاطرة (أو مجازفة) تنطوي على موت الكثير

And if you wish to keep "mass death" intact, you need to play an additional trick:
رغم كون ذلك مخاطرة (أو مجازفة) قد تؤدي إلى موت جماعي