Fudging the time a little... it's still Saturday in my timezone, but only just. Sorry for the delay; house of sick is fun. (Toddlers, yo.)
Young Siward, as you can see, is given up as proof of Mackers' kickassery. In fact, it's the first time he's killed someone on stage - though we've heard of his exploits. You can have such a fight played out for a while, but you'll get one of those in a few moments.
The assumption would be that because he's the son of the valiant English general, Young Siward is therefore a notable warrior in his own right. Maybe so, but he's taken out by Mackers here.
It might just be the angle of photography, but that slash from Macbeth really seemed to stab Siward in a vital part or three! :D
As Dennis once said, "Just remember, age and treachery outdo youth and skill every time." Farewell young Siward.
Young Siward will get his due a little later on, no worries.
As for the number of strikes... Well, the "crunch" in the second-last panel's the overwhelming of Siward's grip on the sword (maybe even bone or sinews,) and the slash in the panel before was mostly to get the sword up and ready for the stab. This is mostly stage choreography - having Mackers' sword back in the early stages permits the audience to see Young Siward's strike and Mackers' shield block before he goes on the offensive.