Would someone who views sexually explicit images and videos when hacking blackmailers' devices in order to delete them be considered sinful, by virtue of necessity?

If you were unaware of the presence of forbidden images in a video clip, there is no sin in accessing it. However, if these images appear, you must delete the clip. What your eyes inadvertently fall upon at first glance from such images falls under the category of "a sudden glance," for which there is no sin. Nevertheless, you must avert your gaze immediately, based on the Almighty's saying: "Tell the believing men to lower their gaze," and the Prophet's (peace be upon him) saying: "So he commanded me to avert my gaze." However, if you are aware of the presence of forbidden images in a clip, it is not permissible to watch it, and it must be deleted.