No you won't kill them. Such drastic pruning is often an unfortunate necessity when shrubs get way too large. Try to cut large branches back to side shoots rather than just stubbing them off. This will not alway be possible but is worthwhile as it helps redirect regrowth into a more natural form.
After pruning they'll resprout and take off growing vigorously again. Be ready to do some minor training through the year to direct and control this regrowth. Some of the many new shoots that will emerge from a stubbed cut branch end will need to be removed to thin out the shrub and rebuild a more natural open branched form.
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