The most important detail to focus on, is (per comment 12 by Brian Trzupek above) that Trustwave knew when it issued the certificate that it would be used to sign certificates for websites not owned by Trustwave’s corporate customer.
That is, Trustwave sold a certificate knowing that it would be used to perform active man-in-the-middle interception of HTTPS traffic.
This is very very different than the usual argument that is used to justify “legitimate” intermediate certificates: the corporate customer wants to generate lots of certs for internal servers that it owns.
Regardless of the fact that Trustwave has since realized that this is not a good business practice to be engaged in, the damage is done.
With root certificate power comes great responsibility. Trustwave has abused this power and trust, and so the appropriate punishment here is death (of its root certificate).
~Christopher Soghoian, in comment about Trustwave