I can't speak for No1, but Tsarnev was convicted under 18 U.S.C. § 2332a – Use of a Weapon of Mass Destruction. This seems to have a much stronger federal connection than the kidnapping and murder of an individual, however reprehensible the latter may be.
What does or should constitute a 'weapon of mass destruction'?
"The scope and usage of the term has evolved and been disputed,
often signifying more politically than technically."
The Boston Marathon bombing involved two crude home-made explosive devices,
which killed three people. Why should these bombs be regarded as WMDs
when some much more lethal weapons used by mass murderers are not?
Stephen Paddock used guns (including assault-type rifles) to kill 58 people.
But his weapons would not be regarded as WMDs.
I would submit that Stephen Paddock's arsenal of guns was much more
dangerous than two crude home-made explosive devices (supposed WMDs).
"This seems to have a much stronger federal connection ..."
Why? It seems to me that a 'federal connection' was based upon the victim
being from abroad, thus lending the case an international dimension.
By the way, the Boston Marathon bombing killed Lu Lingzi, a female graduate
student (in mathematics and statistics) from China.
"....the kidnapping and murder of an individual."
The prosecution's apparently arguing that the death penalty is justified because
Brendt Christiansen also raped and tortured Zhang Yingying before finally killing her.