Because she knows the truth Roy's father kill her fathers.
If we take a look at the subtitles:
I know who you are.
- You are Clifford McBride's son.
- Well, then we're both victims
of the Lima Project.
- My parents left for that expedition.
- You and I share a great loss, Roy.
- Please, just tell me.
ROY: You were right.
The Surge is coming
from the Lima Project.
is very possible.
SPACECOM has determined
that I am unfit to carry on.
That's all I know.
Helen: I believe your father is alive.
Helen: They never told you what
happened to him out there?
Roy: did they?
She shows to Roy a clasified video:
U.S. Space Command - Top Secret Briefing.
Communications has received the following mayday call...from the crew of the Lima Project under the command of H. Clifford McBride. This is Clifford McBride, reporting from the Lima Project. I'm disclosing a tragedy. Here, on the edge of our solar system, some of our people...have been unable to handle the psychological distress...of being so far away from home. They desired to return to Earth, and I could not permit that. And I have to report the reality that they mutinied...committed acts of sabotage trying to commandeer my ship. I was forced to react with equal severity. I disabled one section of our station's life support system...and without doubt, I did punish the innocent along with the guilty.
HE KILL ALL THE CREW
We will not turn back. We will venture further into space. We will find alien intelligence. I am forever driven on this quest.
END OF THE VIDEO.
HELEN: SPACECOM, would never allow their image to be shattered...so they made him a hero to protect themselves.
Helen: Your father murdered my parents. HERE IS THE MAIN MOTIVATION. That monster threatens us all. SECOND MOTIVATION. And now it's your burden. Can you get me on that ship? I can get you to the rocket. After that, you're on your own. I will deal with him. I will deal with my father.
And Helen has enough reputation on Mars to do that (Helen: I am responsible for more than 1,100 people here) SPACECOM is a private enteprise from earth and sometimes people have to take that risk.