They think that the character of whole life would be comprised if they were not to hasten their death in certain circumstances，should they so choose.

该句结构分析如下：

1.  think 之后的宾语从句  the character of whole life would be comprised if they were not to hasten their death in certain circumstances，是一个虚拟语气句。

2.  should they so choose 跟宾语从句无关，而是与主句they think 有关。它是they think 的条件状语从句。

3.  should they so choose 是省略了if 的倒装语序，正常语序是：

If they should so choose。

4.  should 用于条件句，不一定是虚拟语气，而是对将来的一种可能性推测。主句用陈述语气。

5.  关于should 用于if 从句，这种特殊用法见《柯林斯词典》：

MODAL (用在条件从句中表示事件发生的可能性)将要，假定要

You use should in conditional clauses when you are talking about things that might happen.

If you should be fired, your health and pension benefits will not be automatically cut off.

如果你被解雇，你的医疗和养老保险金不会自动中断。

Should you buy a home from Lovell, the company promises to buy it back at the same price after three years.

如果你从洛弗尔公司买房子，该公司承诺3年后以同样的价格再购回。

为了理解方便，我们把should they so choose 提到句首，跟they think 紧挨着。原句等于：

1. Should they so choose, they think that the character of whole life would be comprised if they were not to hasten their death in certain circumstances.

2. If they should so choose, they think that the character of whole life would be comprised if they were not to hasten their death in certain circumstances.

【翻译】 他们若是这样选择的话，他们就会认为，如果他们在某些情况下不加快死亡的速度，那么整个生命的特征就会形成。