@breizh
They were plotting to kill the chief because he doesn't really care about the employees' well-being (no insurance, high risk low pay, ...). The plan was that Roku-san (old guy) had to "accidentally" drop that metal sheet on the chief to kill him. But Ryouko who didn't know anything about it, saved the chief and died as a result.
Roku-san volunteered to do it because he's the oldest (didn't have a lot to live for, so even if he gets some big sentence, it wouldn't really matter anymore because he'll probably die from old age within a few years).
The whole group probably never wished to involve Ryouko-chan because she really likes the chief (cuz he saved him and all), but she became the number one victim instead.
The Japanese kept it semi-ambiguous so I translated it like that too.