The Heat-Ray used by the Martians is utterly deadly. It appears harmlessly enough as a beam of light and is silent, but anyone or anything it touches is instantly consumed and charred by heat.
It is the use of this Heat-Ray, on the curious crowds grouped around the Martian cylinder, that brings home the realisation of how deadly and merciless the Martians actually are. Before this, the Martians appear to pose little or no threat as they are slow and ponderous and seemingly unable even to get out of their cylinder. With the Heat-Ray, however, the whole situation changes in an instant, although this change is not immediately grasped by the narrator:
I stood staring, not as yet realising that this was death leaping from man to man in that little distant crowd. (Part I, chapter 5)
Very soon, however, he is forced to flee for his life.
This is the beginning of the real drama, the wholly unequal struggle between the humans who appear utterly puny and helpless in the face of the Martians’ frighteningly advanced technology. The humans muster all their resources against the Martians’ vastly superior weaponry, but to no avail.
In the end, it is not technology but nature which prove to be humanity’s great ally and, indeed, the wholesale conquerors of the Martians: Earth’s bacteria, against which these extra-terrestrial marauders simply have no protection.